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We present an analysis of spectroscopic data of the cool, highly magnetic, and polluted white dwarf 2MASS J0916−4215. The atmosphere of the white dwarf is dominated by hydrogen, but numerous spectral lines of magnesium, calcium, titanium, chromium, iron, and strontium, along with Li  i , Na  i , Al  i , and K  i lines, are found in the incomplete Paschen–Back regime, most visibly, in the case of Ca  ii lines. Extensive new calculations of the Paschen–Back effect in several spectral lines are presented and results of the calculations are tabulated for the Ca  ii H&K doublet. The abundance pattern shows a large lithium and strontium excess, which may be viewed as a signature of planetary debris akin to Earth's continental crust accreted on to the star, although the scarcity of silicon indicates possible dilution in bulk Earth material. Accurate abundance measurements proved sensitive to the value of the broadening parameter due to collisions with neutral hydrogen (⁠|$\Gamma$| H  i ), particularly in saturated lines such as the resonance lines of Ca  i and Ca  ii. We found that |$\Gamma$| H  i if formulated with values from the literature could be overestimated by a factor of 10 in most resonance lines. [ABSTRACT FROM AUTHOR]

There is a wealth of evidence to suggest that planetary systems can survive beyond the main sequence. Most commonly, white dwarfs are found to be accreting material from tidally disrupted asteroids, whose bulk compositions are reflected by the metals polluting the stellar photospheres. While many examples are known, most lack the deep, high-resolution data required to detect multiple elements, and thus characterize the planetesimals that orbit them. Here, spectra of seven DZ white dwarfs observed with Keck High Resolution Echelle Spectrometer (HIRES) are analysed, where up to nine metals are measured per star. Their compositions are compared against those of Solar system objects, working in a Bayesian framework to infer or marginalize over the accretion history. All of the stars have been accreting primitive material, similar to chondrites, with hints of a Mercury-like composition at one star. The most polluted star is observed several Myr after its last major accretion episode, in which a Moon-sized object met its demise. [ABSTRACT FROM AUTHOR]

Two recently discovered white dwarfs, WD J041246.84 + 754942.26 and WD J165335.21 − 100116.33, exhibit Hα and Hβ Balmer line emission similar to stars in the emerging DAHe class, yet intriguingly have not been found to have detectable magnetic fields. These white dwarfs are assigned the spectral type DAe. We present detailed follow-up of the two known DAe stars using new time-domain spectroscopic observations and analysis of the latest photometric time-series data from TESS and ZTF. We measure the upper magnetic field strength limit of both stars as B < 0.05 MG. The DAe white dwarfs exhibit photometric and spectroscopic variability, where in the case of WD J041246.84 + 754942.26 the strength of the Hα and Hβ emission cores varies in antiphase with its photometric variability over the spin period, which is the same phase relationship seen in DAHe stars. The DAe white dwarfs closely cluster in one region of the Gaia Hertzsprung–Russell diagram together with the DAHe stars. We discuss current theories on non-magnetic and magnetic mechanisms which could explain the characteristics observed in DAe white dwarfs, but additional data are required to unambiguously determine the origin of these stars. [ABSTRACT FROM AUTHOR]

Author(s): John J. Tobin (corresponding author) [1, 2]; Kaitlin M. Kratter [3]; Magnus V. Persson [2, 4]; Leslie W. Looney [5]; Michael M. Dunham [6]; Dominique Segura-Cox [5]; Zhi-Yun Li [...]

We present observations and analyses of eight white dwarf stars (WDs) that have accreted rocky material from their surrounding planetary systems. The spectra of these helium-atmosphere WDs contain detectable optical lines of all four major rock-forming elements (O, Mg, Si, and Fe). This work increases the sample of oxygen-bearing WDs with parent body composition analyses by roughly 33%. To first order, the parent bodies that have been accreted by the eight WDs are similar to those of chondritic meteorites in relative elemental abundances and oxidation states. Seventy-five percent of the WDs in this study have observed oxygen excesses implying volatiles in the parent bodies with abundances similar to those of chondritic meteorites. Three WDs have oxidation states that imply more reduced material than found in CI chondrites, indicating the possible detection of Mercury-like parent bodies, but are less constrained. These results contribute to the recurring conclusion that extrasolar rocky bodies closely resemble those in our solar system, and do not, as a whole, yield unusual or unique compositions. [ABSTRACT FROM AUTHOR]

Ultraviolet and optical spectra of the hydrogen-dominated atmosphere white dwarf star G238-44 obtained with FUSE, Keck/HIRES, HST/COS, and HST/STIS reveal 10 elements heavier than helium: C, N, O, Mg, Al, Si, P, S, Ca, and Fe. G238-44 is only the third white dwarf with nitrogen detected in its atmosphere from polluting planetary system material. Keck/HIRES data taken on 11 nights over 24 yr show no evidence for variation in the equivalent width of measured absorption lines, suggesting stable and continuous accretion from a circumstellar reservoir. From measured abundances and limits on other elements, we find an anomalous abundance pattern and evidence for the presence of metallic iron. If the pollution is from a single parent body, then it would have no known counterpart within the solar system. If we allow for two distinct parent bodies, then we can reproduce the observed abundances with a mix of iron-rich Mercury-like material and an analog of an icy Kuiper Belt object with a respective mass ratio of 1.7:1. Such compositionally disparate objects would provide chemical evidence for both rocky and icy bodies in an exoplanetary system and would be indicative of a planetary system so strongly perturbed that G238-44 is able to capture both asteroid and Kuiper Belt–analog bodies near-simultaneously within its <100 Myr cooling age. [ABSTRACT FROM AUTHOR]

Polluted white dwarfs (WDs) offer a unique way to study the bulk compositions of exoplanetary material, but it is not always clear if this material originates from comets, asteroids, moons, or planets. We combine N -body simulations with an analytical model to assess the prevalence of extrasolar moons as WD polluters. Using a sample of observed polluted WDs, we find that the extrapolated parent body masses of the polluters are often more consistent with those of many solar system moons, rather than solar-like asteroids. We provide a framework for estimating the fraction of WDs currently undergoing observable moon accretion based on results from simulated WD planetary and moon systems. Focusing on a three-planet WD system of super-Earth to Neptune-mass bodies, we find that we could expect about one percent of such systems to be currently undergoing moon accretions as opposed to asteroid accretion. [ABSTRACT FROM AUTHOR]

V488 Persei is the most extreme debris disk known in terms of the fraction of the stellar luminosity it intercepts and reradiates. The infrared output of its disk is extremely variable, similar in this respect to the most variable disk known previously, that around ID8 in NGC 2547. We show that the variations are likely to be due to collisions of large planetesimals (≳100 km in diameter) in a belt being stirred gravitationally by a planetary or low-mass-brown-dwarf member of a planetary system around the star. The dust being produced by the resulting collisions is falling into the star due to drag by the stellar wind. The indicated planetesimal destruction rate is so high that it is unlikely that the current level of activity can persist for much longer than ∼1000–10,000 yr and it may signal a major realignment of the configuration of the planetary system. [ABSTRACT FROM AUTHOR]

WD 0145+234 is a white dwarf that is accreting metals from a circumstellar disc of planetary material. It has exhibited a substantial and sustained increase in 3–5  |$\mu$| m flux since 2018. Follow-up Spitzer photometry reveals that emission from the disc had begun to decrease by late 2019. Stochastic brightening events superimposed on the decline in brightness suggest the liberation of dust during collisional evolution of the circumstellar solids. A simple model is used to show that the observations are indeed consistent with ongoing collisions. Rare emission lines from circumstellar gas have been detected at this system, supporting the emerging picture of white dwarf debris discs as sites of collisional gas and dust production. [ABSTRACT FROM AUTHOR]

We present follow-up photometry and spectroscopy of ZTF J0328−1219, strengthening its status as a white dwarf exhibiting transiting planetary debris. Using TESS and Zwicky Transient Facility photometry, along with follow-up high-speed photometry from various observatories, we find evidence for two significant periods of variability at 9.937 and 11.2 hr. We interpret these as most likely the orbital periods of different debris clumps. Changes in the detailed dip structures within the light curves are observed on nightly, weekly, and monthly timescales, reminiscent of the dynamic behavior observed in the first white dwarf discovered to harbor a disintegrating asteroid, WD 1145+017. We fit previously published spectroscopy along with broadband photometry to obtain new atmospheric parameters for the white dwarf, with M⋆ = 0.731 ± 0.023 M⊙, Teff = 7630 ± 140 K, and [Ca/He] = − 9.55 ± 0.12. With new high-resolution spectroscopy, we detect prominent and narrow Na D absorption features likely of circumstellar origin, with velocities 21.4 ± 1.0 km s−1 blueshifted relative to atmospheric lines. We attribute the periodically modulated photometric signal to dusty effluents from small orbiting bodies such as asteroids or comets, but we are unable to identify the most likely material that is being sublimated, or otherwise ejected, as the environmental temperatures range from roughly 400 to 700 K. [ABSTRACT FROM AUTHOR]

The element beryllium is detected for the first time in white dwarf stars. This discovery in the spectra of two helium-atmosphere white dwarfs was made possible only because of the remarkable overabundance of Be relative to all other elements, heavier than He, observed in these stars. The measured Be abundances, relative to chondritic, are by far the largest ever seen in any astronomical object. We anticipate that the Be in these accreted planetary bodies was produced by spallation of one or more of O, C, and N in a region of high fluence of particles of MeV or greater energy. [ABSTRACT FROM AUTHOR]

Planets and stars ultimately form out of the collapse of the same cloud of gas. Whilst planets, and planetary bodies, readily loose volatiles, a common hypothesis is that they retain the same refractory composition as their host star. This is true within the Solar system. The refractory composition of chondritic meteorites, Earth, and other rocky planetary bodies are consistent with solar, within the observational errors. This work aims to investigate whether this hypothesis holds for exoplanetary systems. If true, the internal structure of observed rocky exoplanets can be better constrained using their host star abundances. In this paper, we analyse the abundances of the K-dwarf, G200-40, and compare them to its polluted white dwarf companion, WD 1425+540. The white dwarf has accreted planetary material, most probably a Kuiper belt-like object, from an outer planetary system surviving the star's evolution to the white dwarf phase. Given that binary pairs are chemically homogeneous, we use the binary companion, G200-40, as a proxy for the composition of the progenitor to WD 1425+540. We show that the elemental abundances of the companion star and the planetary material accreted by WD 1425+540 are consistent with the hypothesis that planet and host-stars have the same true abundances, taking into account the observational errors. [ABSTRACT FROM AUTHOR]

Optical spectroscopic observations of white dwarf stars selected from catalogs based on the Gaia DR2 database reveal nine new gaseous debris disks that orbit single white dwarf stars, about a factor of 2 increase over the previously known sample. For each source we present gas emission lines identified and basic stellar parameters, including abundances for lines seen with low-resolution spectroscopy. Principle discoveries include (1) the coolest white dwarf (Teff ≈ 12,720 K) with a gas disk; this star, WD0145+234, has been reported to have undergone a recent infrared outburst; (2) co-location in velocity space of gaseous emission from multiple elements, suggesting that different elements are well mixed; (3) highly asymmetric emission structures toward SDSS J0006+2858, and possibly asymmetric structures for two other systems; (4) an overall sample composed of approximately 25% DB and 75% DA white dwarfs, consistent with the overall distribution of primary atmospheric types found in the field population; and (5) never-before-seen emission lines from Na in the spectra of Gaia J0611−6931, semi-forbidden Mg, Ca, and Fe lines toward WD 0842+572, and Si in both stars. The currently known sample of gaseous debris disk systems is significantly skewed toward northern hemisphere stars, suggesting a dozen or so emission line stars are waiting to be found in the southern hemisphere. [ABSTRACT FROM AUTHOR]

We report distinctive core profiles in the strongest optical helium line, He i λ5876, from high-resolution high-sensitivity observations of spectral type DB white dwarfs. By analyzing a sample of 40 stars from Keck/HIRES and VLT/UVES, we find the core appearance to be related to the degree of hydrogen and heavy element content in the atmosphere. New Ca K-line measurements or upper limits are reported for about half the sample stars. He i λ5876 emission cores with a self-reversed central component are present for those stars with relatively low hydrogen abundance, as well as relatively low atmospheric heavy element pollution. This self-reversed structure disappears for stars with higher degrees of pollution and/or hydrogen abundance, giving way to a single absorption core. From our model atmospheres, we show that the self-reversed emission cores can be explained by temperature inversions in the upper atmosphere. We propose that the transition to a single absorption core is due to the additional opacity from hydrogen and heavy elements that inhibits the temperature inversions. Our current models do not exactly match the effective temperature range of the phenomenon or the amplitude of the self-reversed structure, which is possibly a result of missing physics such as 3D treatment, convective overshoot, and/or non-LTE effects. The He i λ5876 line structure may prove to be a useful new diagnostic for calibrating temperature profiles in DB atmosphere models. [ABSTRACT FROM AUTHOR]

We report our long-term spectroscopic monitoring of the Pleiades member HII-2147, which was previously spatially resolved at radio wavelengths in very long baseline interferometry (VLBI) observations. It has also been claimed to be a (presumably short-period) double-lined spectroscopic binary with relatively sharp lines, although no orbit has ever been published. Examination of our new spectroscopic material and the historical radial velocities shows that the current and previous spectra are best interpreted as featuring only a single set of lines of a moderately rapidly rotating star with slowly variable radial velocity, which is one of the sources detected by VLBI. We combine our own and other velocities with the VLBI measurements and new adaptive optics observations to derive the first astrometric-spectroscopic orbit of the G5 + G9 pair, with a period of 18.18 ± 0.11 yr. We infer dynamical masses of 0.897 ± 0.022 M☉ for the spectroscopically visible star and 0.978 ± 0.024 M☉ for the other, along with a distance of pc. The lack of detection of the lines of the more massive component in our spectra can be adequately explained if it is rotating much more rapidly than the star we see. This is consistent with the observation that the lines of the secondary are shallower than expected for a star of its spectral type. [ABSTRACT FROM AUTHOR]

We present multiepoch infrared photometry and spectroscopy obtained with warm Spitzer, Subaru, and the Stratospheric Observatory for Infrared Astronomy to assess variability for the young (∼20 Myr) and dusty debris systems around HD 172555 and HD 113766A. No variations (within 0.5%) were found for the former at either 3.6 or 4.5 μm, while significant nonperiodic variations (peak to peak of ∼10%–15% relative to the primary star) were detected for the latter. Relative to the Spitzer Infrared Spectrograph spectra taken in 2004, multiepoch mid-infrared spectra reveal no change in either the shape of the prominent 10 μm solid-state features or the overall flux levels (no more than 20%) for both systems, corroborating the fact that the population of submicron-size grains that produce the pronounced solid-state features is stable over a decadal timescale. We suggest that these submicron-size grains were initially generated in an optically thick clump of debris of millimeter-size vapor condensates resulting from a recent violent impact between large asteroidal or planetary bodies. Because of the shielding from the stellar photons provided by this clump, intense collisions led to an overproduction of fine grains that would otherwise be ejected from the system by radiation pressure. As the clump is sheared by its orbital motion and becomes optically thin, a population of very fine grains could remain in stable orbits until Poynting–Robertson drag slowly spirals them into the star. We further suggest that the 3–5 μm disk variation around HD 113766A is consistent with a clump/arc of such fine grains on a modestly eccentric orbit in its terrestrial zone. [ABSTRACT FROM AUTHOR]

Main sequence stars hosting extreme quantities of inner planetary system debris are likely experiencing transient dust production events. The nature of these events, if they can be unambiguously attributed to a single process, can potentially inform us on the formation and/or early evolution of rocky Earth-like planets. In this contribution I examine some of the dustiest main sequence stars known and three processes that may be capable of reproducing their observed properties. Through this activity I also make an estimate for the likelihood of an A-type star to have an asteroid belt-like planetesimal population. [ABSTRACT FROM AUTHOR]

We report two new dramatically dusty main sequence stars: HD 131488 (A1 V) and HD 121191 (A8 V). HD 131488 is found to have substantial amounts of dust in its terrestrial planet zone (LIR/Lbol ≈ 4 × 10–3), cooler dust farther out in its planetary system, and an unusual mid-infrared spectral feature. HD 121191 shows terrestrial planet zone dust (LIR/Lbol ≈ 2.3 × 10–3), hints of cooler dust, and shares the unusual mid-infrared spectral shape identified in HD 131488. These two stars belong to sub-groups of the Scorpius-Centaurus OB association and have ages of ∼10 Myr. HD 131488 and HD 121191 are the dustiest main sequence A-type stars currently known. Early-type stars that host substantial inner planetary system dust are thus far found only within the age range of 5-20 Myr. [ABSTRACT FROM AUTHOR]

We report an in-depth study of the F8-type star HD 166191, identified in an ongoing survey for stars exhibiting infrared emission above their expected photospheres in the Wide-field Infrared Survey Explorer all-sky catalog. The fractional IR luminosity measured from 3.5 to 70 μm is exceptionally high (LIR/Lbol ∼ 10%). Near-diffraction-limited imaging observations with the T-ReCS Si filter set on the Gemini South telescope and adaptive optics imaging with the NIRC2 Lp filter on the Keck II telescope confirmed that the excess emission coincides with the star. Si-band images show a strong solid-state emission feature at ∼10 μm. Theoretical evolutionary isochrones and optical spectroscopic observations indicate a stellar age in the range 10-100 Myr. The large dust mass seen in HD 166191's terrestrial planet zone is indicative of a recent collision between planetary embryos or massive ongoing collisional grinding associated with planet building. [ABSTRACT FROM AUTHOR]

We develop a means to distinguish between warm dusty circumstellar debris disks produced by steady state collisional evolution of a planetesimal belt or through transient events. [ABSTRACT FROM PUBLISHER]

The Pleiades is the best-studied open cluster in the sky. It is one of the primary open clusters used to define the ‘zero-age main sequence,' and hence it serves as a cornerstone for programs which use main-sequence fitting to derive distances. This role is called into question by the ‘Pleiades distance controversy:' the distance to the Pleiades from Hipparcos of approximately 120 pc is significantly different from the distance of 133 pc derived using other techniques. To resolve this issue, we plan to use Very Long Baseline Interferometry to derive a new, independent trigonometric parallax distance to the Pleiades. In these proceedings we present our observational program and report some preliminary results. [ABSTRACT FROM PUBLISHER]

We have obtained near-infrared adaptive optics imaging and collected additional radial velocity observations to search for a third component in the extremely dusty short-period binary system BD +20°307. Our image shows no evidence for a third component at separations greater than 19AU. Our four seasons of radial velocities have a constant center-of-mass velocity and are consistent with the systemic velocities determined at two earlier epochs. Thus, the radial velocities also provide no support for a third component. Unfortunately, the separation domains covered by our imaging and radial velocity results do not overlap. Thus, we examined the parameters for possible orbits of a third component that could have been missed by our current observations. With our velocities we determined improved circular orbital elements for the 3.4 day double-lined binary. We also performed a spectroscopic abundance analysis of the short-period binary components and conclude that the stars are a mid- and a late-F dwarf. We find that the iron abundances of both components, [Fe/H] = 0.15, are somewhat greater than the solar value and comparable to that of stars in the Hyades. Despite the similarity of the binary components, the lithium abundances of the two stars are very unequal. The primary has log ε (Li) = 2.72, while in the secondary log ε (Li) ≤ 1.46, which corresponds to a difference of at least a factor of 18. The very disparate lithium abundances in very similar stars make it impossible to ascribe a single age to them. While the system is likely at least 1 Gyr old, it may well be as old as the Sun. [ABSTRACT FROM AUTHOR]

We present the methods and first results of a survey of nearby high proper motion main-sequence stars to probe for cool companions with the Gemini camera at Lick Observatory. This survey uses a sample of old (age > 2 Gyr) stars as targets to probe for companions down to temperatures of 500 K. Multi-epoch observations allow us to discriminate comoving companions from background objects. So far, our survey has successfully rediscovered the wide T8.5 companion to GJ 1263 and has discovered a companion to the nearby M0V star GJ 660.1. The companion to GJ 660.1 (GJ 660.1B) is ~4 mag fainter than its host star in the J-band and is located at a projected separation of ~ 120 AU. Known trigonometric parallax and Two Micron All Sky Survey magnitudes for the GJ 660.1 system indicate a spectral type for the companion of M9 ± 2. [ABSTRACT FROM AUTHOR]

The presence of elements heavier than helium in white dwarf atmospheres is often a signpost for the existence of rocky objects that currently or previously orbited these stars. We have measured the abundances of various elements in the hydrogen-atmosphere white dwarfs G 149-28 and NLTT 43806. In comparison with other white dwarfs with atmospheres polluted by heavy elements, NLTT 43806 is substantially enriched in aluminum but relatively poor in iron. We compare the relative abundances of Al and eight other heavy elements seen in NLTT 43806 with the elemental composition of bulk Earth, with simulated extrasolar rocky planets, with solar system meteorites, with the atmospheric compositions of other polluted white dwarfs, and with the outer layers of the Moon and Earth. The best agreement is found with a model that involves accretion of a mixture of terrestrial crust and upper mantle material onto NLTT 43806. The implication is that NLTT 43806 is orbited by a differentiated rocky planet, perhaps quite similar to Earth, that has suffered a collision that stripped away some of its outer layers. [ABSTRACT FROM AUTHOR]

We present the first large sample of metal absorption systems in pairs of QSOs with sightlines separated by about 1 Mpc at . We found 690 absorption systems in the spectra of 310 QSOs in 170 pairings. Most systems show C iv or Mg ii absorption. When we see absorption in one QSO, the probability that we see absorption in the paired QSO, within about 500 km s−1, is at least ∼50 per cent at <100 kpc, declining rapidly to ∼8 per cent at 200–400 kpc, ∼0.8 per cent by 1–2 Mpc proper distance. Although we may occasionally see an individual absorbing halo in two sightlines, the absorber–absorber correlation is primarily a probe of the distribution of metals around galaxies and Mpc scale galaxy clustering. QSO absorption lines give redshifts errors of ∼23 km s−1, almost 10 times smaller than the error for galaxy spectra at these redshifts, hence we can measure clustering on small scales, around 0.5 Mpc proper, with a small sample. The distribution of 23 absorber–absorber coincidences separated by <2.5 Mpc at is consistent with an origin in galaxies with a normal correlation function, normal systematic infalling velocities and low random pair-wise velocity differences, more consistent with blue than with red galaxies. Absorption in gas flowing out from galaxies with a mean velocity of 250 km s−1 would produce more redshift elongation than we see. The fast winds detected by Adelberger et al. in the same ions account for less than 1/3 of the absorption systems we see. Such winds may be confined to the ultraviolet (UV) luminous star-forming regions of Lyman break galaxies. If most galaxies have winds, they cannot extend to 40 kpc with large velocities, while continuing to make UV absorption that we can detect. This suggests that most metals seen in the intergalactic medium at arrived long before. We see an excess of C iv absorbers, with an a posteriori probability of 0.0003, when a line of sight passes a foreground QSO. We see 16 absorbers where we expect 5.8 at 0–600 km s−1, on the front side of the partner QSO. At these velocities, we see an excess absorber in ∼6 per cent of sightlines that pass within 0.1–2.5 Mpc of a QSO, but in <2 per cent of cases when we look directly at a QSO. These transverse associated absorbers are not the normal line-of-sight associated absorbers that have a broader velocity distribution. Excluding the sightlines to us, the 3D distribution of 59 absorbers around 313 QSOs is approximately isotropic, except for the 1.5–2σ tendency for the excess C iv absorbers to be on the front side of the QSOs. Our QSO redshifts may be too large by ∼300 km s−1, or there might be a real asymmetry coming from a hypothetical anisotropy in the QSO UV emission, or from isotropic UV emission that lasted less than ∼1 Myr, possibilities suggested by the excess H i behind these QSOs. The velocity dispersion of the excess absorbers near the QSOs is small, ∼250 km s−1, suggesting that both these absorbers and the QSOs are in the blue sequence of galaxies. The probability of seeing absorption when a sightline passes a QSO rises only slowly as the impact parameter drops from 2.5 to 0.1 Mpc, perhaps because the UV radiation from QSOs destroys many nearby absorbers. [ABSTRACT FROM AUTHOR]

We present 40 fully hydrodynamical numerical simulations of the intergalactic gas that gives rise to the Lyα forest. The simulation code, input and output files are available at . For each simulation we predict the observable properties of the H i absorption in quasar or quasi-stellar object (QSO) spectra. We then find the sets of cosmological and astrophysical parameters which result in spectra whose properties match that of the QSO spectra. We present our results as scaling relationships between input and output parameters. The input parameters include the main cosmological parameters and σ8; and two astrophysical parameters and X228. The parameter controls the rate of ionization of H i, He i and He ii and is equivalent to the intensity of the ultraviolet background (UVB). The second parameter X228 controls the rate of heating from the photoionization of He ii and can be related to the shape of the UVB at Å. We show how these input parameters, especially and X228, affect the output parameters that we measure in simulated spectra. These parameters are the mean flux , a measure of the most common (as defined more precisely in ) Lyα linewidth ( b-value) , and the one-dimensional power spectrum of the flux on scales from 0.01 to 0.1 s km−1. We compare the simulation output with data from Kim et al. and Tytler et al., and we give a new measurement of the flux power from HIRES and UVES spectra for the low-density intergalactic medium (IGM) alone at . We find that simulations with a wide variety of σ8-values, from at least 0.8 to 1.1, can fit the small-scale flux power and b-values when we adjust X228 to compensate for the σ8 change. We can also use to adjust the H i ionization rate to match the mean flux simultaneously. When we examine only the mean flux, b-values and small-scale flux power we cannot readily break the strong degeneracy between σ8 and X228. We can break the degeneracy using large-scale flux power or other data to fix σ8. When we pick a specific σ8-value the simulations give the value of X228 that we need to match the observed small-scale flux power and b-values. We can then also find the required to match the mean flux for that combination of σ8 and X228. We derive scaling relations that give the output parameter values expected for a variety of input parameters. We predict the linewidth parameter with an error of 1.4 per cent and the mean amount of H i absorption to 2 per cent, equivalent to a 0.27 per cent error on at . These errors are four times smaller than those on the best current measurement. We can readily calculate the sets of input parameters that give outputs that match the data. For , with and , we find and , equivalent to ionizations per H i atom per second. If we run an optically thin simulation with these parameters in a box size of 76.8 Mpc comoving and with a cell size of 18.75 kpc comoving, we expect that the simulated spectra will match Lyα forest data at . The rates predicted by Madau, Haardt & Rees correspond to and . Our results for match while the larger X228 is reasonable to correct for the opacity that is missing from the optically thin simulations. For a smaller value of σ8 the structures are generally more extended and we need a smaller X228 corresponding to a cooler IGM, as found by Bryan & Machacek (their fig. 7). We also need a larger to stop the neutral fraction from increasing at the lower temperatures. [ABSTRACT FROM AUTHOR]

We use high-quality echelle spectra of 24 quasi-stellar objects to provide a calibrated measurement of the total amount of Lyα forest absorption (DA) over the redshift range . Our measurement of DA excludes absorption from metal lines or the Lyα lines of Lyman-limit systems and damped Lyα systems. We use artificial spectra with realistic flux calibration errors to show that we are able to place continuum levels that are accurate to better than 1 per cent. When we combine our results with our previous results between , we find that the redshift evolution of DA is well described over as , where and . We detect no significant deviations from a smooth power-law evolution over the redshift range studied. We find less H i absorption than expected at , implying that the ultraviolet background is per cent higher than expected. Our data appears to be consistent with an H i ionization rate of . [ABSTRACT FROM AUTHOR]

We present continuum and molecular line observations at 230 and 345 GHz from the Submillimeter Array (SMA) toward three protostars in the Perseus L1448N region. The data are from the large project "Mass Assembly of Stellar Systems and their Evolution with the SMA." Three dust continuum sources, Source B, Source NW, and Source A, are detected at both frequencies. These sources have corresponding emission peaks in C18O ($J=2\to 1$), 13CO ($J=2\to 1$), and HCO+ ($J=4\to 3$), and have offsets with N2D+ ($J=3\to 2$) peaks. High angular resolution data from a complementary continuum survey with the Karl G. Jansky Very Large Array show that Source B is associated with three 8 mm continuum objects, Source NW with two, and Source A remains single. These results suggest that multiplicity in L1448N exists at different spatial scales from a few thousand AU to <100 AU. Velocity gradients in each source obtained from two-dimensional fits to the SMA C18O emission are found to be perpendicular to within 20° of the outflow directions as revealed by 12CO ($J=2\to 1$). We have observed that Sources B and NW with multiplicity have higher densities than Source A without multiplicity. This suggests that thermal Jeans fragmentation can be relevant in the fragmentation process. However, we have not observed a difference in the ratio between rotational and gravitational energy between sources with and without multiplicity. We also have not observed a trend between non-thermal velocity dispersions and the level of fragmentation. Our study has provided the first direct and comprehensive comparison between multiplicity and core properties in low-mass protostars, although based on small number statistics., Astronomy

We report the detection of radio emission from the unusually active L5e + T7 binary 2MASS J13153094−2649513AB made with the Australian Telescope Compact Array. Observations at 5.5 GHz reveal an unresolved source with a continuum flux of 370±50 µJy, corresponding to a radio luminosity of Lrad = νLν = (9±3)×1023 erg s−1 and log10 Lrad/Lbol = −5.44±0.22. No detection is made at 9.0 GHz to a 5σ limit of 290 µJy, consistent with a power law spectrum Sν ∝ ν−α with α & 0.5. The emission is quiescent, with no evidence of variability or bursts over 3 hr of observation, and no measurable polarization (V/I < 34%). 2MASS J1315−2649AB is one of the most radio-luminous ultracool dwarfs detected in quiescent emission to date, comparable in strength to other cool sources detected in outburst. Its detection indicates no decline in radio flux through the mid-L dwarfs. It is unique among L dwarfs in having strong and persistent Hα and radio emission, indicating the coexistence of a cool, neutral photosphere (low electron density) and a highly active chromosphere (high electron density and active heating). These traits, coupled with the system’s mature age and substellar secondary, makes 2MASS J1315−2649AB an important test for proposed radio emission mechanisms in ultracool dwarfs., Astronomy

We are conducting a Jansky Very Large Array (VLA) Ka-band (8 mm and 1 cm) and C-band (4 cm and 6.4 cm) survey of all known protostars in the Perseus Molecular Cloud, providing resolution down to ∼0.″06 and ∼0.″35 in the Ka band and C band, respectively. Here we present first results from this survey that enable us to examine the source NGC 1333 IRAS2A in unprecedented detail and resolve it into a protobinary system separated by 0.″621 ± 0.″006 (∼143 AU) at 8 mm, 1 cm, and 4 cm. These two sources (IRAS2A VLA1 and VLA2) are likely driving the two orthogonal outflows known to originate from IRAS2A. The brighter source IRAS2A VLA1 is extended perpendicular to its outflow in the VLA data, with a deconvolved size of 0.″055 (∼13 AU), possibly tracing a protostellar disk. The recently reported candidate companions (IRAS2A MM2 and MM3) are not detected in either our VLA data, Combined Array for Research in Millimeter-wave Astronomy (CARMA) 1.3 mm data, or Submillimeter Array (SMA) 850 μm data. SMA CO (J = 3 → 2), CARMA CO (J = 2 → 1), and lower-resolution CARMA CO (J = 1 → 0) observations are used to examine the outflow origins and the nature of the candidate companions to IRAS2A VLA1. The CO (J = 3 → 2) and (J = 2 → 1) data show that IRAS2A MM2 is coincident with a bright CO emission spot in the east-west outflow, and IRAS2A MM3 is within the north-south outflow. In contrast, IRAS2A VLA2 lies at the east-west outflow symmetry point. We propose that IRAS2A VLA2 is the driving source of the east-west outflow and a true companion to IRAS2A VLA1, whereas IRAS2A MM2 and MM3 may not be protostellar. [ABSTRACT FROM AUTHOR]

It has recently been shown that a significant fraction of late-type members of nearby, very young associations (age ≲10 Myr) display excess emission at mid-IR wavelengths indicative of dusty circumstellar disks. We demonstrate that the detection of mid-IR excess emission can be utilized to identify new nearby, young, late-type stars including two definite new members (“TWA 33” and “TWA 34”) of the TW Hydrae Association (TWA). Both new TWA members display mid-IR excess emission in the Wide-field Infrared Survey Explorer catalog and they show proper motion and youthful spectroscopic characteristics—namely, Hα emission, strong lithium absorption, and low surface gravity features consistent with known TWA members. We also detect mid-IR excess—the first unambiguous evidence of a dusty circumstellar disk—around a previously identified UV-bright, young, accreting star (2M1337) that is a likely member of the Lower-Centaurus Crux region of the Scorpius-Centaurus Complex. [ABSTRACT FROM AUTHOR]

We assess the current membership of the nearby, young TW Hydrae association and examine newly proposed members with the Wide-field Infrared Survey Explorer (WISE) to search for infrared excess indicative of circumstellar disks. Newly proposed members TWA 30A, TWA 30B, TWA 31, and TWA 32 all show excess emission at 12 and 22 μm providing clear evidence for substantial dusty circumstellar disks around these low-mass, ∼8 Myr old stars that were previously shown to likely be accreting circumstellar material. TWA 30B shows large amounts of self-extinction, likely due to an edge-on disk geometry. We also confirm previously reported circumstellar disks with WISE and determine a 22 μm excess fraction of 42+10– 9% based on our results. [ABSTRACT FROM AUTHOR]

Because of its proximity to the Galactic plane, reliable identification of members of the α Persei cluster is often problematic. Based primarily on membership evaluations contained in six published papers, we constructed a mostly complete list of high-fidelity members of spectral type G and earlier that lie within 3 arc degrees of the cluster center. α Persei was the one nearby, rich, young open cluster not surveyed with the Spitzer Space Telescope. We examined the first and final data releases of the Wide-field Infrared Survey Explorer and found 11, or perhaps 12, α Per cluster members that have excess mid-infrared emission above the stellar photosphere attributable to an orbiting dusty debris disk. The most unusual of these is V488 Per, a K-type star with an excess IR luminosity 16% (or more) of the stellar luminosity; this is a larger excess fraction than that of any other known dusty main-sequence star. Much of the dust that orbits V488 Per is at a temperature of ∼800 K; if these grains radiate like blackbodies, then they lie only ∼0.06 AU from the star. The dust is probably the aftermath of a collision of two planetary embryos or planets with small semimajor axes; such orbital radii are similar to those of many of the transiting planets discovered by the Kepler satellite. [ABSTRACT FROM AUTHOR]

With adaptive optics imaging at Keck observatory, we have discovered a substellar companion to the F6 Pleiades star HD 23514, one of the dustiest main-sequence stars known to date (LIR/L* ∼ 2%). This is one of the first brown dwarfs discovered as a companion to a star in the Pleiades. The 0.06 M☼ late-M secondary has a projected separation of ∼360 AU. The scarcity of substellar companions to stellar primaries in the Pleiades combined with the extremely dusty environment make this a unique system to study. [ABSTRACT FROM AUTHOR]