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Classical Cepheids are essential objects in the study of stellar evolution and cosmology; however, we know little about their magnetic properties. We report the detection of Stokes $V$ features interpreted as Zeeman signatures in four classical Cepheids using high-resolution spectropolarimetric observations obtained with ESPaDOnS at CFHT. Eight observations of $\eta$ Aql were acquired in 2017 covering its 7.2 d pulsation period, and single observations of Polaris, $\zeta$ Gem, $\delta$ Cep and RT Aur were obtained in 2020 as part of our ongoing systematic survey. We use mean circular polarization Stokes $V$ profiles generated using the Least-Squares Deconvolution procedure to diagnose Zeeman signatures and measure mean longitudinal field strengths $\langle B_{z}\rangle$. We detect magnetic signatures across all pulsation phases of $\eta$ Aql ($-0.89\pm0.47$ G$\,<\langle B_{z}\rangle<1.27\pm 0.40$ G), as well as in the single observations of Polaris ($0.59\pm0.16$ G), $\zeta$ Gem ($0.41\pm0.16$ G) and $\delta$ Cep ($0.43\pm0.19$ G). The Stokes $V$ profile of Polaris is detected at extremely high S/N and implies a complex magnetic field topology. It stands in stark contrast to all other detected Stokes $V$ profiles, which show unusual approximately unipolar positive circular polarization lobes analogous to those observed in some Am stars., Comment: 11 pages, 6 figures, accepted for publication in MNRAS

We present H-band interferometric observations of the red supergiant (RSG) AZ Cyg made with the Michigan Infra-Red Combiner (MIRC) at the six-telescope Center for High Angular Resolution Astronomy (CHARA) Array. The observations span 5 years (2011-2016), offering insight into the short and long-term evolution of surface features on RSGs. Using a spectrum of AZ Cyg obtained with SpeX on the NASA InfraRed Telescope Facility (IRTF) and synthetic spectra calculated from spherical MARCS, spherical PHOENIX, and SAtlas model atmospheres, we derive $T_{\text{eff}}$ is between $3972 K$ and $4000 K$ and $\log~g$ between $-0.50$ and $0.00$, depending on the stellar model used. Using fits to the squared visibility and Gaia parallaxes we measure its average radius $R=911^{+57}_{-50}~R_{\odot}$. Reconstructions of the stellar surface using our model-independent imaging codes SQUEEZE and OITOOLS.jl show a complex surface with small bright features that appear to vary on a timescale of less than one year and larger features that persist for more than one year. 1D power spectra of these images suggest a characteristic size of $0.52-0.69~R_{\star}$ for the larger, long lived features. This is close to the values of $0.51-0.53~R_{\star}$ derived from 3D RHD models of stellar surfaces. We conclude that interferometric imaging of this star is in line with predictions of 3D RHD models but that short-term imaging is needed to more stringently test predictions of convection in RSGs., Comment: 31 pages, 8 tables, 14 figures, accepted to ApJ

We study the polarization produced by scattering from dust in a bow shock-shaped region of enhanced density surrounding a stellar source, using the Monte Carlo radiative transfer code SLIP. Bow shocks are structures formed by the interaction of the winds of fast-moving stars with the interstellar medium. Our previous study focused on the polarization produced in these structures by electron scattering; we showed that polarization is highly dependent on inclination angle and that multiple scattering changes the shape and degree of polarization. In contrast to electron scattering, dust scattering is wavelength-dependent, which changes the polarization behaviour. Here we explore different dust particle sizes and compositions and generate polarized spectral energy distributions for each case. We find that the polarization SED behaviour depends on the dust composition and grain size. Including dust emission leads to polarization changes with temperature at higher optical depth in ways that are sensitive to the orientation of the bow shock. In various scenarios and under certain assumptions, our simulations can constrain the optical depth and dust properties of resolved and unresolved bow shock-shaped scattering regions.Constraints on optical depth can provide estimates of local ISM density for observed bow shocks. We also study the impact of dust grains filling the region between the star and bow shock. We see that as the density of dust between the star and bow shock increases, the resulting polarization is suppressed for all the optical depth regimes., Comment: 21 pages, accepted for publication in MNRAS

Cepheids are pulsating variable stars with a periodic chromospheric response at UV wavelengths close to their minimum radius phase. Recently, an X-ray variable signature was captured in observations during the \emph{maximum} radius phase. This X-ray emission came as a surprise and is not understood. In this work, we use the modern astrophysical code, PLUTO, to investigate the effects of pulsations on Cepheid X-ray emission. We run a number of hydrodynamic numerical simulations with a variety of initial and boundary conditions in order to explore the capability of shocks to produce the observed phase-dependent X-ray behavior. Finally we use the Simulated Observations of X-ray Sources (SOXS) package to create synthetic spectra for each simulation case and link our simulations to observables. We show that, for certain conditions, we can reproduce observed X-ray fluxes at phases 0.4--0.8 when the Cepheid is at maximum radius. Our results span a wide range of mass-loss rates, $2\times10^{-13}$--$3\times10^{-8}$ $M_\odot$ yr$^{-1}$, and peak X-ray luminosities, $5\times10^{-17}$--$1.4\times10^{-12}$ erg cm$^{-2}$ s$^{-1}$. We conclude that Cepheids exhibit \textit{two component emission} with (a) shock waves being responsible for the phase dependent variable emission (phases 0.2 - 0.6), and (b) a separate quiescent mechanism being the dominant emission mechanism for the remaining phases.

Polaris is the nearest Cepheid to us and as such holds a special place in our understanding of Cepheids in general and the Leavitt Law. In the past couple of decades, we have learned many new things about the star as a Cepheid and as the primary component of a multiple star system. As such, we are more precisely measuring the mass, radius and evolution of Polaris. However, as we learn more, it is becoming clear that we understand less. There is evidence that Polaris is much less massive than stellar evolution models suggest and that Polaris is a much younger star than its main sequence companion. In this work, we review some of the recent measurements and their connections with past studies. We then present new stellar evolution models and populations synthesis calculations to compare with the new mass measurements by Evans et al. (2018). We find that the mass discrepancy for Polaris is about 50\%. We also find that there is a likely age discrepancy between Polaris and its companion, but that there is also a very small probability that the discrepancy is not real., Comment: 5 pages, 1 figure, submitted for Astronomical Society of the Pacific Conference Series "RRLyrae/Cepheid2019: Frontiers of Classical Pulsators: Theory and Observations", Cloudcroft, NM, USA October 13-18, 2019, Corrected references

This white paper explains that professional astronomy has benefited from settler colonial white supremacist patriarchy. We explicate the impact that this has had on communities which are not the beneficiaries of colonialism and white supremacy. We advocate for astronomers to reject these benefits in the future, and we make proposals regarding the steps involved in rejecting colonialist white supremacy's benefits. We center ten recommendations on the timely issue of what to do about the Thirty Meter Telescope (TMT) on Maunakea in Hawaii. This paper is written in solidarity with and support of efforts by Native Hawaiian scientists (e.g. Kahanamoku et al. 2019)., Comment: 8 pages, APC (State of the Profession) White Paper submitted to the Astro2020 Decadal Survey

(Abridged) Canadian astronomy has, for decades, benefited from access to observatories and participating in international consortia on one of the best astronomical sites in the world: Maunakea. However, Maunakea is part of the unceded territory of the Native Hawaiian peoples and has always been of special significance to Hawaiian culture. The use of the summit and its science reserve has created tensions in the past decade, particularly with the development of the Thirty Meter Telescope. A meaningful and respectful response from the International astronomy community is still lacking. It is expected that the LRP 2020 will continue to support Canadian astronomy on Maunakea so a better official statement on the position and involvement of CASCA should be prepared. In this paper we present recommendations, based on the United Nation Declaration for the Rights of Indigenous Peoples, for the Canadian astronomical community to better support Indigenous rights on Maunakea and Hawaii while providing clear guidelines for the astronomical community to participate in activities conducted on Indigenous land. This framework is designed to motivate conversations with Indigenous communities regarding our place on Indigenous lands and our roles, and responsibilities toward the communities we are working with. Furthermore, we propose this framework as a basis for engaging with communities around the world regarding consent for astronomical facilities., Comment: 7 pages. Community paper submitted to the Canadian Long Range Plan 2020, https://casca.ca/?page_id=11499lrp2020/

(Abridged) The Truth and Reconciliation Commission of Canada published its calls to action in 2015 with 94 recommendations. Many of these 94 recommendations are directly related to education, language, and culture, some of which the Canadian Astronomy community can address and contribute to as part of reconciliation. The Canadian Astronomy community has an additional obligation since it benefits from facilities on Indigenous territories across Canada and the world. Furthermore, Indigenous people are still underrepresented at all levels in Canadian astronomy. The purpose of this Community Paper is to develop recommendations for the Canadian astronomy community to support Indigenous inclusion in the science community, support Indigenous learning by developing Indigenous-based learning materials and facilitate access to professionals and science activities, and to recognize and acknowledge the great contributions of Indigenous communities to our science activities. As part of this work we propose the ten following recommendations for CASCA as an organization and throughout this Community Paper we will include additional recommendations for individuals: astronomers, students and academics., Comment: 11 pages. Community paper submitted to the Canadian Long Range Plan 2020, https://casca.ca/?page_id=11499lrp2020/

(Abridged) The Maunakea Spectroscopic Explorer (MSE) is an end-to-end science platform for the design, execution and scientific exploitation of spectroscopic surveys. It will unveil the composition and dynamics of the faint Universe and impact nearly every field of astrophysics across all spatial scales, from individual stars to the largest scale structures in the Universe. Major pillars in the science program for MSE include (i) the ultimate Gaia follow-up facility for understanding the chemistry and dynamics of the distant Milky Way, including the outer disk and faint stellar halo at high spectral resolution (ii) galaxy formation and evolution at cosmic noon, via the type of revolutionary surveys that have occurred in the nearby Universe, but now conducted at the peak of the star formation history of the Universe (iii) derivation of the mass of the neutrino and insights into inflationary physics through a cosmological redshift survey that probes a large volume of the Universe with a high galaxy density. MSE is positioned to become a critical hub in the emerging international network of front-line astronomical facilities, with scientific capabilities that naturally complement and extend the scientific power of Gaia, the Large Synoptic Survey Telescope, the Square Kilometer Array, Euclid, WFIRST, the 30m telescopes and many more., Comment: 9 chapters, 301 pages, 100 figures. This version of the DSC is a comprehensive update of the original version, released in 2016, which can be downloaded at arXiv:1606.00043. A detailed summary of the design of MSE is available in the MSE Book 2018, available at arXiv:1810.08695

The Maunakea Spectroscopic Explorer (MSE) is a planned 11.25-m aperture facility with a 1.5 square degree field of view that will be fully dedicated to multi-object spectroscopy. A rebirth of the 3.6m Canada-France-Hawaii Telescope on Maunakea, MSE will use 4332 fibers operating at three different resolving powers (R ~ 2500, 6000, 40000) across a wavelength range of 0.36-1.8mum, with dynamical fiber positioning that allows fibers to match the exposure times of individual objects. MSE will enable spectroscopic surveys with unprecedented scale and sensitivity by collecting millions of spectra per year down to limiting magnitudes of g ~ 20-24 mag, with a nominal velocity precision of ~100 m/s in high-resolution mode. This white paper describes science cases for stellar astrophysics and exoplanet science using MSE, including the discovery and atmospheric characterization of exoplanets and substellar objects, stellar physics with star clusters, asteroseismology of solar-like oscillators and opacity-driven pulsators, studies of stellar rotation, activity, and multiplicity, as well as the chemical characterization of AGB and extremely metal-poor stars., Comment: 31 pages, 11 figures; To appear as a chapter for the Detailed Science Case of the Maunakea Spectroscopic Explorer

Classical Cepheids are powerful probes of both stellar evolution and near-field cosmology thanks to their high luminosities, pulsations, and that they follow the Leavitt (Period-Luminosity) Law. However, there still exist a number of questions regarding their evolution, such as the role of rotation, convective core overshooting and winds. ln particular, how do these processes impact Cepheid evolution and the predicted fundamental properties such as stellar mass. In this work, we compare a sample of period change that are real-time observations of stellar evolution with new evolution models to test the impact of these first two processes. In our previous study we found that enhanced mass loss is crucial for describing the sample, and here we continue that analysis but for rotational mixing and core overshooting. We show that, while rotation is important for stellar evolution studies, rotation, itself, is insufficient to model the distribution of period change rates from the observed sample. On the other hand, convective core overshooting is needed to explain the magnitude of the rates of period change, but does not explain the number of stars with positive and negative period change rates. In conclusion, we determine that convective core overshooting and stellar rotation alone are not enough to account for the observed distribution of Cepheid rates of period change and another mechanism, such as pulsation-driven mass-loss, may be required., Comment: 8 pages, 4 figures, 2 tables, accepted for publication in AAS journals

The ability to observe extrasolar planets transiting their stars has profoundly changed our understanding of these planetary systems. However, these measurements depend on how well we understand the properties of the host star, such as radius, luminosity and limb darkening. Traditionally, limb darkening is treated as a parameterization in the analysis, but these simple parameterizations are not accurate representations of actual center-to-limb intensity variations (CLIV) to the precision needed for interpreting these transit observations. This effect leads to systematic errors for the measured planetary radii and corresponding measured spectral features. We compute synthetic planetary transits using model stellar atmosphere CLIV and corresponding best-fit limb-darkening laws for a grid spherically symmetric model stellar atmospheres. From these light curves we measure the differences in flux as a function of the star's effective temperature, gravity, mass, and the inclination of the planet's orbit., Comment: 10 pages, 8 figures, submitted to AAS journals. Comments welcome

Bow shocks and related density enhancements produced by the winds of massive stars moving through the interstellar medium provide important information regarding the motions of the stars, the properties of their stellar winds, and the characteristics of the local medium. Since bow shocks are aspherical structures, light scattering within them produces a net polarization signal even if the region is spatially unresolved. Scattering opacity arising from free electrons and dust leads to a distribution of polarized intensity across the bow shock structure. That polarization encodes information about the shape, composition, opacity, density, and ionisation state of the material within the structure. In this paper we use the Monte Carlo radiative transfer code SLIP to investigate the polarization created when photons scatter in a bow shock-shaped region of enhanced density surrounding a stellar source. We present results assuming electron scattering, and investigate the polarization behaviour as a function of optical depth, temperature, and source of photons for two different cases: pure scattering and scattering with absorption. In both regimes we consider resolved and unresolved cases. We discuss the implication of these results as well as their possible use along with observational data to constrain the properties of observed bow shock systems. In different situations and under certain assumptions, our simulations can constrain viewing angle, optical depth and temperature of the scattering region, and the relative luminosities of the star and shock., Comment: 20 pages, 13 figures, 1 table; Accepted for publication in MNRAS

The transit method, employed by MOST, \emph{Kepler}, and various ground-based surveys has enabled the characterization of extrasolar planets to unprecedented precision. These results are precise enough to begin to measure planet atmosphere composition, planetary oblateness, star spots, and other phenomena at the level of a few hundred parts-per-million. However, these results depend on our understanding of stellar limb darkening, that is, the intensity distribution across the stellar disk that is sequentially blocked as the planet transits. Typically, stellar limb darkening is assumed to be a simple parameterization with two coefficients that are derived from stellar atmosphere models or fit directly. In this work, we revisit this assumption and compute synthetic planetary transit light curves directly from model stellar atmosphere center-to-limb intensity variations (CLIV) using the plane-parallel \textsc{Atlas} and spherically symmetric \textsc{SAtlas} codes. We compare these light curves to those constructed using best-fit limb-darkening parameterizations. We find that adopting parametric stellar limb-darkening laws lead to systematic differences from the more geometrically realistic model stellar atmosphere CLIV of about 50 -- 100 ppm at the transit center and up to 300 ppm at ingress/egress. While these errors are small they are systematic, and appear to limit the precision necessary to measure secondary effects. Our results may also have a significant impact on transit spectra., Comment: 12 pages, 14 figures, accepted for publication in ApJ after revisions

We review ways in which observations of the changing periods of pulsating variable stars can be used to detect and directly measure their evolution. We briefly describe the two main techniques of analysis -- (O-C) analysis and wavelet analysis -- and results for pulsating variable star types which are reasonably periodic: type I and II Cepheids, RR Lyrae stars, beta Cephei stars, and Mira stars. We comment briefly on delta Scuti stars and pulsating white dwarfs. For some of these variable star types, observations agree approximately with the predictions of evolutionary models, but there still exist significant areas of disagreement that challenge future models of stellar evolution. There may be a need, for instance, to include processes such as rotation, mass loss, and magnetic fields. There may also be non-evolutionary processes which are contributing to the period changes., Comment: To be published in the Journal of the American Association of Variable Star Observers

One of the great challenges in understanding stars is measuring their masses. The best methods for measuring stellar masses include binary interaction, asteroseismology and stellar evolution models, but these methods are not ideal for red giant and supergiant stars. In this work, we propose a novel method for inferring stellar masses of evolved red giant and supergiant stars using interferometric and spectrophotometric observations combined with spherical model stellar atmospheres to measure what we call the stellar mass index, defined as the ratio between the stellar radius and mass. The method is based on the correlation between different measurements of angular diameter, used as a proxy for atmospheric extension, and fundamental stellar parameters. For a given star, spectrophotometry measures the Rosseland angular diameter while interferometric observations generally probe a larger limb-darkened angular diameter. The ratio of these two angular diameters is proportional to the relative extension of the stellar atmosphere, which is strongly correlated to the star's effective temperature, radius and mass. We show that these correlations are strong and can lead to precise measurements of stellar masses., Comment: 7 pages, 3 figures, accepted for publication in ApJ

The classical Cepheid $l$ Carinae is an essential calibrator of the Cepheid Leavitt Law as a rare long-period Galactic Cepheid. Understanding the properties of this star will also constrain the physics and evolution of massive ($M \ge 8$ $M_\odot$) Cepheids. The challenge, however, is precisely measuring the star's pulsation period and its rate of period change. The former is important for calibrating the Leavitt Law and the latter for stellar evolution modeling. In this work, we combine previous time-series observations spanning more than a century with new observations to remeasure the pulsation period and compute the rate of period change. We compare our new rate of period change with stellar evolution models to measure the properties of $l$ Car, but find models and observations are, at best, marginally consistent. The results imply that $l$ Car does not have significantly enhanced mass-loss rates like that measured for $\delta$ Cephei. We find that the mass of $l$ Car is about 8 - 10 $M_\odot$. We present Hubble Space Telescope COS observations that also differ from measurements for $\delta$ Cep, and $\beta$ Dor. These measurements further add to the challenge of understanding the physics of Cepheids, but do hint at the possible relation between enhanced mass loss and ultraviolet emission, perhaps both due to the strength of shocks propagating in the atmospheres of Cepheids., Comment: 11 pages, 6 figures, 1 table, ApJ accepted

The $\beta$ Cephei stars represent an important class of massive star pulsators probing the evolution of B-type stars and the transition from main sequence to hydrogen-shell burning evolution. By understanding $\beta$ Cep stars, we gain insights into the detailed physics of massive star evolution such as rotational mixing, convective core overshooting, magnetic fields and stellar winds, all of which play important roles. Similarly, modeling their pulsation provides additional information into their interior structures. Furthermore, measurements of the rate of change of pulsation period offer a direct measure of $\beta$ Cephei stellar evolution. In this work, we compute state-of-the-art stellar evolution models assuming different amounts of initial rotation and convective core overshoot and measure theoretical rates of period change for which we compare to rates previously measured for a sample of $\beta$ Cephei stars. The results of this comparison are mixed. For three stars, the rates are too small to infer any information from stellar evolution models, whereas for three other stars the rates are too large. We infer stellar parameters, such as mass and age, for two $\beta$ Cephei stars: $\xi^1$ CMa and $\delta$ Cet, that agree well with independent measurements. We explore ideas for why models may not predict the larger rates of period change. In particular, period drifts in $\beta$ Cep stars can artificially lead to overestimated rates of secular period change., Comment: 6 pages, 2 figures 2 tables, accepted to A&A

Classical Cepheids are key probes of both stellar astrophysics and cosmology as standard candles and pulsating variable stars. It is important to understand Cepheids in unprecedented detail in preparation for upcoming GAIA, JWST and extremely-large telescope observations. Cepheid eclipsing binary stars are ideal tools for achieving this goal, however there are currently only three known systems. One of those systems, OGLE-LMC-CEP1812, raises new questions about the evolution of classical Cepheids because of an apparent age discrepancy between the Cepheid and its red giant companion. We show that the Cepheid component is actually the product of a stellar merger of two main sequence stars that has since evolved across the Hertzsprung gap of the HR diagram. This post-merger product appears younger than the companion, hence the apparent age discrepancy is resolved. We discuss this idea and consequences for understanding Cepheid evolution., Comment: 5 pages, 3 figures, accepted to A&A

Classical Cepheids, like binary stars, are laboratories for stellar evolution and Cepheids in binary systems are especially powerful ones. About one-third of Galactic Cepheids are known to have companions and Cepheids in eclipsing binary systems have recently been discovered in the Large Magellanic Cloud. However, there are no known Galactic binary Cepheids with orbital periods less than one year. We compute population synthesis models of binary Cepheids to compare to the observed period and eccentricity distributions of Galactic Cepheids as well as to the number of observed eclipsing binary Cepheids in the LMC. We find that our population synthesis models are consistent with observed binary properties of Cepheids. Furthermore, we show that binary interaction on the red giant branch prevents some red giant stars from becoming classical Cepheids. Such interactions suggest that the binary fraction of Cepheids should be significantly less than that of their main-sequence progenitors, and that almost all binary Cepheids have orbital periods longer than one year. If the Galactic Cepheid spectroscopic binary fraction is about 35%, then the spectroscopic binary fraction of their intermediate mass main sequence progenitors is about 40-45%., Comment: 7 pages, 3 figures, resubmitted to A&A

We present results from a study of the eclipsing, colliding-wind binary V444 Cyg that uses a combination of X-ray and optical spectropolarimetric methods to describe the 3-D nature of the shock and wind structure within the system. We have created the most complete X-ray light curve of V444 Cyg to date using 40 ksec of new data from Swift, and 200 ksec of new and archived XMM-Newton observations. In addition, we have characterized the intrinsic, polarimetric phase-dependent behavior of the strongest optical emission lines using data obtained with the University of Wisconsin's Half-Wave Spectropolarimeter. We have detected evidence of the Coriolis distortion of the wind-wind collision in the X-ray regime, which manifests itself through asymmetric behavior around the eclipses in the system's X-ray light curves. The large opening angle of the X-ray emitting region, as well as its location (i.e. the WN wind does not collide with the O star, but rather its wind) are evidence of radiative braking/inhibition occurring within the system. Additionally, the polarimetric results show evidence of the cavity the wind-wind collision region carves out of the Wolf-Rayet star's wind., Comment: 19 pages, 14 figures, accepted A&A

Over the past decade, the Secret Lives of Cepheids (SLiC) program has been carried out at Villanova University to study aspects and behaviors of classical Cepheids that are still not well-understood. In this, the first of several planned papers on program Cepheids, we report the current results for delta Cep, the Cepheid prototype. Ongoing photometry has been obtained to search for changes in the pulsation period, light curve morphology and amplitude. Combining our photometry with the times of maximum light compilation by Berdnikov 2000 returns a small period change of dP/dt ~ -0.1006 +/- 0.0002 sec yr^-1. There is also evidence for a gradual light amplitude increase of ~0.011-mag (V-band) and ~0.012-mag (B-band) per decade over the last ~50 years. In addition, HST-COS UV spectrophotometry and XMM-Newton X-ray data were carried out to investigate the high-temperature plasmas present above the Cepheid photospheres. In total, from the five visits (eight exposures) with XMM-Newton, delta Cep is found to be a soft X-ray source (Lx [0.3--2 keV] ~ 4.5-13x10^28 erg sec^-1) with peak flux at kT = 0.6-0.9 keV. The X-ray activity is found to vary, possibly in phase with the stellar pulsations. From 2010-2013, nine observations of delta Cep were carried out with HST-COS. The UV emissions are also variable, and well-phased with the stellar pulsations. Maximum UV line emissions occur near, or slightly before, maximum optical light, varying by as much as twenty times. This variability shows that pulsation-induced shock-heating plays a significant role in Cepheid atmospheres, possibly in addition to a quiescent, magnetic heating. The results of this study show Cepheid atmospheres to be rather complex and dynamic., Comment: 34 pages, 7 figures, 4 tables - To appear in Engle et al. 2014, ApJ, 794, 80

Betelgeuse, a nearby red supergiant, is a runaway star with a powerful stellar wind that drives a bow shock into its surroundings. This picture has been challenged by the discovery of a dense and almost static shell that is three times closer to the star than the bow shock and has been decelerated by some external force. The two physically distinct structures cannot both be formed by the hydrodynamic interaction of the wind with the interstellar medium. Here we report that a model in which Betelgeuse's wind is photoionized by radiation from external sources can explain the static shell without requiring a new understanding of the bow shock. Pressure from the photoionized wind generates a standing shock in the neutral part of the wind and forms an almost static, photoionization-confined shell. Other red supergiants should have significantly more massive shells than Betelgeuse, because the photoionization-confined shell traps up to 35 per cent of all mass lost during the red supergiant phase, confining this gas close to the star until it explodes. After the supernova explosion, massive shells dramatically affect the supernova lightcurve, providing a natural explanation for the many supernovae that have signatures of circumstellar interaction., Comment: 22 pages, 8 figures, authors' version, accepted for publication in Nature; minor differences may exist compared with published version; final version available at http://dx.doi.org/10.1038/nature13522

We discuss two important effects for the astrospheres of runaway stars: the propagation of ionizing photons far beyond the astropause, and the rapid evolution of massive stars (and their winds) near the end of their lives. Hot stars emit ionizing photons with associated photoheating that has a significant dynamical effect on their surroundings. 3D simulations show that HII regions around runaway O stars drive expanding conical shells and leave underdense wakes in the medium they pass through. For late O stars this feedback to the interstellar medium is more important than that from stellar winds. Late in life, O stars evolve to cool red supergiants more rapidly than their environment can react, producing transient circumstellar structures such as double bow shocks. This provides an explanation for the bow shock and linear bar-shaped structure observed around Betelgeuse., Comment: 5 pages, 2 figures, conference proceedings in "Wind Bubbles, Astrospheres and the Heliosphere: Environments and Cosmic Rays", Bochum, Germany (Nov 2013). Published in ASTRA proceedings, an Open Access Journal for Refereed Proceedings in Extraterrestrial Research

Measurements of secular period change probe real-time stellar evolution of classical Cepheids making these measurements powerful constraints for stellar evolution models, especially when coupled with interferometric measurements. In this work, we present stellar evolution models and measured rates of period change for two Galactic Cepheids: Polaris and l Carinae, both important Cepheids for anchoring the Cepheid Leavitt law (period-luminosity relation). The combination of previously-measured parallaxes, interferometric angular diameters and rates of period change allows for predictions of Cepheid mass loss and stellar mass. Using the stellar evolution models, We find that l Car has a mass of about 9 $M_\odot$ consistent with stellar pulsation models, but is not undergoing enhanced stellar mass loss. Conversely, the rate of period change for Polaris requires including enhanced mass-loss rates. We discuss what these different results imply for Cepheid evolution and the mass-loss mechanism on the Cepheid instability strip., Comment: 2 pages, 1 figure, Poster presented at IAU307: New windows on massive stars: asteroseismology, interferometry, and spectropolarimetry, Editors: G. Meynet, C. Georgy, J.H. Groh & Ph. Stee

Polarization is a powerful tool for understanding stellar atmospheres and circumstellar environments. Mira and semi-regular variable stars have been observed for decades and some are known to be polarimetrically variable, however, the semi-regular variable V Canes Venatici displays an unusually large, unexplained amount of polarization. We present ten years of optical polarization observations obtained with the HPOL instrument, supplemented by published observations spanning a total interval of about forty years for V CVn. We find that V CVn shows large polarization variations ranging from 1 - 6%. We also find that for the past forty years the position angle measured for V CVn has been virtually constant suggesting a long-term, stable, asymmetric structure about the star. We suggest that this asymmetry is caused by the presence of a stellar wind bow shock and tail, consistent with the star's large space velocity., Comment: 8 pages, 5 figures, 1 table. Accepted for publication in A&A

The circumstellar medium around massive stars is strongly impacted by stellar winds, radiation, and explosions. We use numerical simulations of these interactions to constrain the current properties and evolutionary history of various stars by comparison with observed circumstellar structures. Two- and three-dimensional simulations of bow shocks around red supergiant stars have shown that Betelgeuse has probably only recently evolved from a blue supergiant to a red supergiant, and hence its bow shock is very young and has not yet reached a steady state. We have also for the first time investigated the magnetohydrodynamics of the photoionised H II region around the nearby runaway O star Zeta Oph. Finally, we have calculated a grid of models of bow shocks around main sequence and evolved massive stars that has general application to many observed bow shocks, and which forms the basis of future work to model the explosions of these stars into their pre-shaped circumstellar medium., Comment: 7 pages, 7 figures, page format edited to save space, conference proceedings: NIC Symposium, 12-13 February 2014, Juelich, Germany (Eds. K. Binder, G. Muenster, M. Kremer)

Polaris the Cepheid has been observed for centuries, presenting surprises and changing our view of Cepheids and stellar astrophysics, in general. Specifically, understanding Polaris helps anchor the Cepheid Leavitt law, but the distance must be measured precisely. The recent debate regarding the distance to Polaris has raised questions about its role in calibrating the Leavitt law and even its evolutionary status. In this work, I present new stellar evolution models of Cepheids to compare with previously measured CNO abundances, period change and angular diameter. Based on the comparison, I show that Polaris cannot be evolving along the first crossing of the Cepheid instability strip and cannot have evolved from a rapidly-rotating main sequence star. As such, Polaris must also be at least 118 pc away and pulsates in the first overtone, disagreeing with the recent results of Turner et al. (2013)., Comment: 6 pages, 5 figures, resubmitted to A&A after minor revisions

Analyses of recent observations of the sole classical Cepheid in the Kepler field, V1154 Cygni, found random changes of about 30 minutes in the pulsation period. These period changes challenge standard theories of pulsation and evolution because the period change is non-secular, and explaining this period jitter is necessary for understanding stellar evolution and the role of Cepheids as precise standard candles. We suggest that convection and convective hot spots can explain the observed period jitter. Convective hot spots alter the timing of flux maximum and minimum in the Cepheid light curve, hence change the measured pulsation period. We present a model of random hot spots that generate a localized flux excess that perturbs the Cepheid light curve and consequently the pulsation period which is consistent with the observed jitter. This result demonstrates how important understanding convection is for modeling Cepheid stellar structure and evolution, how convection determines the red edge of the instability strip, and just how sensitive Cepheid light curves are to atmospheric physics., Comment: 5 pages, 4 figures, accepted for publication in A&A Letters

Both pulsation and mass loss are commonly observed in stars and are important ingredients for understanding stellar evolution and structure, especially for massive stars. There is a growing body of evidence that pulsation can also drive and enhance mass loss in massive stars and that pulsation-driven mass loss is important for stellar evolution. In this review, I will discuss recent advances in understanding pulsation driven mass loss in massive main sequence stars, classical Cepheids and red supergiants and present some challenges remaining., Comment: 8 pages, 1 figure, review presented at IAU Symposium 301, Precision Asteroseismology, August 2013, Wroclaw, Poland, corrected errors in reference list

Mira and semi-regular variable stars have been studied for centuries but continue to be enigmatic. One unsolved mystery is the presence of polarization from these stars. In particular, we present 40 years of polarization measurements for the prototype o Ceti and V CVn and find very different phenomena for each star. The polarization fraction and position angle for Mira is found to be small and highly variable. On the other hand, the polarization fraction for V CVn is large and variable, from 2 - 7 %, and its position angle is approximately constant, suggesting a long-term asymmetric structure. We suggest a number of potential scenarios to explain these observations., Comment: 2 pages, 1 figure, poster presented at IAU Symposium 301, Precision Asteroseismology, August 2013, Wroclaw, Poland

Classical Cepheids form one of the foundations of modern cosmology and the extragalactic distance scale, however, cosmic microwave background observations measure cosmological parameters and indirectly the Hubble Constant, H0, to unparalleled precision. The coming decade will provide opportunities to measure H0 to 2% uncertainty thanks to the GAIA satellite, JWST, ELTs and other telescopes using Cepheids and other standard candles. In this work, we discuss the upcoming role for variable stars and asteroseismology in calibrating the distance scale and measuring H0 and what problems exist in understanding these stars that will feedback on these measurements., Comment: 8 pages, summary of splinter session at IAU Symposium 301, Precision Asteroseismology, August 2013, Wroclaw, Poland

Limb darkening is a fundamental ingredient for interpreting observations of planetary transits, eclipsing binaries, optical/infrared interferometry and microlensing events. However, this modeling traditionally represents limb darkening by a simple law having one or two coefficients that have been derived from plane-parallel model stellar atmospheres, which has been done by many researchers. More recently, researchers have gone beyond plane-parallel models and considered other geometries. We previously studied the limb-darkening coefficients from spherically symmetric and plane-parallel model stellar atmospheres for cool giant and supergiant stars, and in this investigation we apply the same techniques to FGK dwarf stars. We present limb-darkening coefficients, gravity-darkening coefficients and interferometric angular diameter corrections from Atlas and SAtlas model stellar atmospheres. We find that sphericity is important even for dwarf model atmospheres, leading to significant differences in the predicted coefficients., Comment: 9 pages, 8 figures. Accepted for publication in A&A

Model stellar atmospheres are fundamental tools for understanding stellar observations from interferometry, microlensing, eclipsing binaries and planetary transits. However, the calculations also include assumptions, such as the geometry of the model. We use intensity profiles computed for both plane-parallel and spherically symmetric model atmospheres to determine fitting coefficients in the BVRIHK, CoRot and Kepler wavebands for limb darkening using several different fitting laws, for gravity-darkening and for interferometric angular diameter corrections. Comparing predicted variables for each geometry, we find that the spherically symmetric model geometry leads to different predictions for surface gravities log g < 3. In particular, the most commonly used limb-darkening laws produce poor fits to the intensity profiles of spherically symmetric model atmospheres, which indicates the need for more sophisticated laws. Angular diameter corrections for spherically symmetric models range from 0.67 to 1, compared to the much smaller range from 0.95 to 1 for plane-parallel models., Comment: 11 pages, 9 figures, accepted for publication in A&A, electronic tables will be available from CDS

The nearby red supergiant (RSG) Betelgeuse has a complex circumstellar medium out to at least 0.5 parsecs from its surface, shaped by its mass-loss history within the past 0.1 Myr, its environment, and its motion through the interstellar medium (ISM). In principle its mass-loss history can be constrained by comparing hydrodynamic models with observations. Observations and numerical simulations indicate that Betelgeuse has a very young bow shock, hence the star may have only recently become a RSG. To test this possibility we calculated a stellar evolution model for a single star with properties consistent with Betelgeuse. We incorporated the resulting evolving stellar wind into 2D hydrodynamic simulations to model a runaway blue supergiant (BSG) undergoing the transition to a RSG near the end of its life. The collapsing BSG wind bubble induces a bow shock-shaped inner shell which at least superficially resembles Betelgeuse's bow shock, and has a similar mass. Surrounding this is the larger-scale retreating bow shock generated by the now defunct BSG wind's interaction with the ISM. We investigate whether this outer shell could explain the bar feature located (at least in projection) just in front of Betelgeuse's bow shock., Comment: 5 pages, 3 figures; to appear in proceedings of the Betelgeuse 2012 Workshop, Paris, Nov. 2012

Mass is the most important stellar parameter, but it is not directly observable for a single star. Spherical model stellar atmospheres are explicitly characterized by their luminosity ($L_\star$), mass ($M_\star$) and radius ($R_\star$), and observations can now determine directly $L_\star$ and $R_\star$. We computed spherical model atmospheres for red giants and for red supergiants holding $L_\star$ and $R_\star$ constant at characteristic values for each type of star but varying $M_\star$, and we searched the predicted flux spectra and surface-brightness distributions for features that changed with mass. For both stellar classes we found similar signatures of the star's mass in both the surface-brightness distribution and the flux spectrum. The spectral features have been use previously to determine $\log_{10} (g)$, and now that the luminosity and radius of a non-binary red giant or red supergiant can be observed, spherical model stellar atmospheres can be used to determine the star's mass from currently achievable spectroscopy. The surface-brightness variations with mass are slightly smaller than can be resolved by current stellar imaging, but they offer the advantage of being less sensitive to the detailed chemical composition of the atmosphere., Comment: 24 pages, 9 figures

Measurements of rates of period change of Classical Cepheids probe stellar physics and evolution. Additionally, better understanding of Cepheid structure and evolution provides greater insight into their use as standard candles and tools for measuring the Hubble constant. Our recent study of the period change of the nearest Cepheid, Polaris, suggested that it is undergoing enhanced mass loss when compared to canonical stellar evolution model predictions. In this work, we expand the analysis to rates of period change measured for about 200 Galactic Cepheids and compare them to population synthesis models of Cepheids including convective core overshooting and enhanced mass loss. Rates of period change predicted from stellar evolution models without mass loss do not agree with observed rates whereas including enhanced mass loss yields predicted rates in better agreement with observations. This is the first evidence that enhanced mass loss as suggested previously for Polaris and delta Cephei must be a ubiquitous property of Classical Cepheids., Comment: 6 pages, 4 figures, Accepted for publication in ApJ Letters

Context. Limb darkening is an important tool for understanding stellar atmospheres, but most observations measuring limb darkening assume various parameterizations that yield no significant information about the structure of stellar atmospheres. Aims. We use a specific limb-darkening relation to study how the best-fit coefficients relate to fundamental stellar parameters from spherically symmetric model stellar atmospheres. Methods. Using a grid of spherically symmetric Atlas model atmospheres, we compute limb-darkening coefficients, and develop a novel method to predict fundamental stellar parameters. Results. We find our proposed method predicts the mass of stellar atmosphere models given only the radius and limb-darkening coefficients, suggesting that microlensing, interferometric, transit and eclipse observations can constrain stellar masses. Conclusions. This novel method demonstrates that limb-darkening parameterizations contain important information about the structure of stellar atmospheres, with the potential to be a valuable tool for measuring stellar masses., Comment: 8 pages, 6 figures, 2 tables, A&A accepted

The projection factor (p), which converts the radial velocity to pulsational velocity, is an important parameter in the Baade-Wesselink (BW) type analysis and distance scale work. The p-factor is either adopted as a constant or linearly depending on the logarithmic of pulsating periods. The aim of this work is to calibrate the p-factor if a Cepheid has both the BW distance and an independent distance measurement, and examine the p-factor for delta Cephei -- the prototype of classical Cepheids. We calibrated the p-factor for several Galactic Cepheids that have both the latest BW distances and independent distances either from Hipparcos parallaxes or main-sequence fitting distances to Cepheid-hosted stellar clusters. Based on 25 Cepheids, the calibrated p-factor relation is consistent with latest p-factor relation in literature. The calibrated p-factor relation also indicates that this relation may not be linear and may exhibit an intrinsic scatter. We also examined the discrepancy of empirical p-factors for delta Cephei, and found that the reasons for this discrepancy include the disagreement of angular diameters, the treatment of radial velocity data, and the phase interval adopted during the fitting procedure. Finally, we investigated the impact of the input p-factor in two BW methodologies for delta Cephei, and found that different p-factors can be adopted in these BW methodologies and yet result in the same angular diameters., Comment: 6 pages, 6 figures and 2 tables. A&A accepted

A significant fraction of massive stars are moving supersonically through the interstellar medium (ISM), either due to disruption of a binary system or ejection from their parent star cluster. The interaction of their wind with the ISM produces a bow shock. In late evolutionary stages these stars may undergo rapid transitions from red to blue and vice versa on the Hertzsprung-Russell diagram, with accompanying rapid changes to their stellar winds and bow shocks. Recent 3D simulations of the bow shock produced by the nearby runaway red supergiant (RSG) Betelgeuse, under the assumption of a constant wind, indicate that the bow shock is very young (<30000 years old), hence Betelgeuse may have only recently become a RSG. To test this possibility, we have calculated stellar evolution models for single stars which match the observed properties of Betelgeuse in the RSG phase. The resulting evolving stellar wind is incorporated into 2D hydrodynamic simulations in which we model a runaway blue supergiant (BSG) as it undergoes the transition to a RSG near the end of its life. We find that the collapsing BSG wind bubble induces a bow shock-shaped inner shell around the RSG wind that resembles Betelgeuse's bow shock, and has a similar mass. Surrounding this is the larger-scale retreating bow shock generated by the now defunct BSG wind's interaction with the ISM. We suggest that this outer shell could explain the bar feature located (at least in projection) just in front of Betelgeuse's bow shock., Comment: 6 pages, 5 figures, emulateapj format, accepted to ApJ letters, simulation animations at http://www.astro.uni-bonn.de/~jmackey/research.html#Betelgeuse

Context. One challenge for measuring the Hubble constant using Classical Cepheids is the calibration of the Leavitt Law or period-luminosity relationship. The Baade-Wesselink method for distance determination to Cepheids relies on the ratio of the measured radial velocity and pulsation velocity, the so-called projection factor and the ability to measure the stellar angular diameters. Aims. We use spherically-symmetric model stellar atmospheres to explore the dependence of the p-factor and angular diameter corrections as a function of pulsation period. Methods. Intensity profiles are computed from a grid of plane-parallel and spherically-symmetric model stellar atmospheres using the SAtlas code. Projection factors and angular diameter corrections are determined from these intensity profiles and compared to previous results. Results. Our predicted geometric period-projection factor relation including previously published state-of-the-art hydrodynamical predictions is not with recent observational constraints. We suggest a number of potential resolutions to this discrepancy. The model atmosphere geometry also affects predictions for angular diameter corrections used to interpret interferometric observations, suggesting corrections used in the past underestimated Cepheid angular diameters by 3 - 5%. Conclusions. While spherically-symmetric hydrostatic model atmospheres cannot resolve differences between projection factors from theory and observations, they do help constrain underlying physics that must be included, including chromospheres and mass loss. The models also predict more physically-based limb-darkening corrections for interferometric observations., Comment: 8 pages, 6 figures, 2 tables, accepted for publication in A&A

One of the largest uncertainties for using the Baade-Wesselink method to measure Cepheid distances is the value of the projection factor (p-factor). However, p-factors measured using the IRSB technique and from hydrodynamic models disagree. In this work, we compute spherically-symmetric static model stellar atmospheres and predict a period p-factor relation., Comment: 2 pages, 1 figure, To be published in Astrophysics and Space Science Proceedings series (ASSP). Proceedings of the "20th Stellar Pulsation Conference Series: Impact of new instrumentation and new insights in stellar pulsations", 5-9 September 2011, Granada, Spain

Polaris is one of the most observed stars in the night sky, with recorded observations spanning more than 200 years. From these observations, one can study the real-time evolution of Polaris via the secular rate of change of the pulsation period. However, the measurements of the rate of period change do not agree with predictions from state-of-the-art stellar evolution models. We show that this may imply that Polaris is currently losing mass at a rate of $\dot{M} \approx 10^{-6} M_\odot$ yr$^{-1}$ based on the difference between modeled and observed rates of period change, consistent with pulsation-enhanced Cepheid mass loss. A relation between the rate of period change and mass loss has important implications for understanding stellar evolution and pulsation, and provides insight into the current Cepheid mass discrepancy., Comment: 5 pages, 4 figures, compiled using emulateapj, Accepted for publication in ApJ Letters. Fixed correction in title

Context. The Cepheid mass discrepancy, the difference between masses predicted from stellar evolution and stellar pulsation calculations, is a challenge for the understanding of stellar astrophysics. Recent models of the eclipsing binary Cepheid OGLE-LMC-CEP-0227 have suggested that the discrepancy may be resolved. Aims. We explore for what physical parameters do stellar evolution models agree with the measured properties of OGLE-LMC-CEP0227 and compare to canonical stellar evolution models assuming no convective core overshooting. Methods. We construct state-of-the-art stellar evolution models for varying mass, metallicity, and convective core overshooting and compare the stellar evolution predictions with the observed properties. Results. The observed mass, effective temperature, and radius of the two stars in the binary system are well fit by numerous combinations of physical parameters, suggesting a Cepheid mass discrepancy of 10-20% relative to canonical stellar evolution models. Conclusions. The properties of the observed binary Cepheid suggest that the Cepheid mass discrepancy is still a challenge and requires more specific observations, such as the rate of period change, to better constrain and understand the necessary physics for stellar evolution models to resolve the discrepancy., Comment: 5 pages, 3 figures, A&A accepted

(Abridged) Context. Stellar limb darkening, I({\mu} = cos{\theta}), is an important constraint for microlensing, eclipsing binary, planetary transit, and interferometric observations, but is generally treated as a parameterized curve, such as a linear-plus-square-root law. Many analyses assume limb-darkening coefficients computed from model stellar atmospheres. However, previous studies, using I({\mu}) from plane- parallel models, have found that fits to the flux-normalized curves pass through a fixed point, a common {\mu} location on the stellar disk, for all values of T eff, log g and wavelength. Aims. We study this fixed {\mu}-point to determine if it is a property of the model stellar atmospheres or a property of the limb-darkening laws. Furthermore, we use this limb-darkening law as a tool to probe properties of stellar atmospheres for comparison to limb- darkening observations. Methods. Intensities computed with plane-parallel and spherically-symmetric Atlas models (characterized by the three fundamental parameters L\star, M\star and R\star) are used to reexamine the existence of the fixed {\mu}-point for the parametrized curves. Results. We find that the intensities from our spherical models do not have a fixed point, although the curves do have a minimum spread at a {\mu}-value similar to the parametrized curves. We also find that the parametrized curves have two fixed points, {\mu}1 and {\mu}2, although {\mu}2 is so close to the edge of the disk that it is missed using plane-parallel atmospheres. We also find that the spherically- symmetric models appear to agree better with published microlensing observations relative to plane-parallel models., Comment: 8 pages, 8 figures, figures 4 and 6 have lower resolution. A&A in press

Context. A longstanding challenge for understanding classical Cepheids is the Cepheid mass discrepancy, where theoretical mass estimates using stellar evolution and stellar pulsation calculations have been found to differ by approximately 10 - 20%. Aims. We study the role of pulsation-driven mass loss during the Cepheid stage of evolution as a possible solution to this mass discrepancy. Methods. We computed stellar evolution models with a Cepheid mass-loss prescription and various amounts of convective core overshooting. The contribution of mass loss towards the mass discrepancy is determined using these models, Results. Pulsation-driven mass loss is found to trap Cepheid evolution on the instability strip, allowing them to lose about 5 - 10% of their total mass when moderate convective core overshooting, an amount consistent with observations of other stars, is included in the stellar models. Conclusions. We find that the combination of moderate convective core overshooting and pulsation-driven mass loss can solve the Cepheid mass discrepancy., Comment: 4 pages, 2 figures and 2 tables. Accepted for publication A&A Letters

We explore the role of mass loss and convective core overshoot in the evolution of Classical Cepheids. Stellar evolution models are computed with a recipe for pulsation-driven mass loss and it is found that mass loss alone is unable to account for the long-standing Cepheid mass discrepancy. However, the combination of mass loss and moderate convective core overshooting does provide a solution, bringing the amount of convective core overshooting in Cepheids closer to that found for other stars., Comment: 8 pages, 3 figures. To appear in the proceedings of Cool Stars 16: 16th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun, corrected typo in author list

We present a novel method for measuring the masses of evolved stars from their limb-darkening observations parameterized as a linear-plus-square-root function with two coefficients. The coefficients of the law are related to integrated moments of the intensity and carry information about the extension of the stellar atmosphere, which is correlated to the ratio of the stellar radius to mass, R/M. Here, we show why the limb-darkening law is related to the R/M, and apply this result to limb-darkening observations of the microlensing event EROS-BLG-2000-5., Comment: 2 pages, 1 figure, To appear in the proceedings of Why Galaxies Care About AGB stars II

In this article, we test the hypothesis that Cepheids have infrared excesses due to mass loss. We fit a model using the mass-loss rate and the stellar radius as free parameters to optical observations from the OGLE-III survey and infrared observations from the 2MASS and SAGE data sets. The sample of Cepheids have predicted minimum mass-loss rates ranging from zero to $10^{-8}M_\odot$ $yr^{-1}$, where the rates depend on the chosen dust properties. We use the predicted radii to compute the Period-Radius relation for LMC Cepheids, and to estimate the uncertainty caused by the presence of infrared excess for determining angular diameters with the infrared surface brightness technique. Finally, we calculate the linear and non-linear Period-Luminosity (P-L) relations for the LMC Cepheids at VIJHK + IRAC wavelengths and we find that the P-L relations are consistent with being non-linear at infrared wavelengths, contrary to previous results., Comment: 17 pages, 12 figures, 5 tables, ApJ Accepted

It has been claimed that Period-Luminosity relations derived from infrared observations of Large Magellanic Cloud (LMC) Cepheids are less dependent on the metallicity of the Cepheids. In this work, infrared observations of LMC Cepheids from the SAGE survey are combined with OGLE II optical observation to model and predict mass-loss rates. The mass-loss rates are fit to the data and are predicted to range from about 10^{-12} to 10^{-7}M_\odot/yr; however, the rates depend on the assumed value of the dust-to-gas ratio. By comparing the relations derived from observations to the relations derived from predicted infrared stellar luminosities from the mass-loss model, it is shown mass loss affects the structure and scatter of the infrared Period-Luminosity relation. Mass loss produces shallower slopes of the infrared relations and a lower zero point. There is also evidence for non-linearity in the predicted Period-Luminosity relations, and it is argued that mass loss produces larger infrared excess at lower periods, which affects the slope and zero point, making the PL relations more linear in the wavelength range of 3.6 to 5.8 micron. Because the dust-to-gas ratio is metallicity dependent and mass loss may have a metallicity dependence, infrared Period-Luminosity relations have additional uncertainty due to metallicity., Comment: 11 pages, 10 figures, 1 table. ApJ Accepted