Search

We present the analysis of a peculiar W Virginis (pWVir) type II Cepheid, OGLE-LMC-T2CEP-211 (Ppuls = 9.393 days), in a double-lined binary system (Porb = 242 days), which shed light on virtually unknown evolutionary status and structure of pWVir stars. The dynamical mass of the Cepheid (first ever for a type II Cepheid) is 0.64 ± 0.02 M⊙, and the radius R = 25.1 ± 0.3 R⊙. The companion is a massive (5.67 M⊙) main-sequence star obscured by a disk. Such a configuration suggests a mass transfer in the system history. We found that originally the system ( = 12 days) was composed of 3.5 and 2.8 M⊙ stars, with the current Cepheid being more massive. The system age is now ∼200 Myr, and the Cepheid is almost completely stripped of hydrogen, with helium mass of ∼92% of the total mass. It finished transferring the mass 2.5 Myr ago and is evolving toward lower temperatures passing through the instability strip. Comparison with observations indicates a reasonable 2.7 × 10−8M⊙ yr−1 mass loss from the Cepheid. The companion is most probably a Be main-sequence star with T = 22,000 K and R = 2.5 R⊙. Our results yield a good agreement with a pulsation theory model for a hydrogen-deficient pulsator, confirming the described evolutionary scenario. We detected a two-ring disk (Rdisk ∼ 116 R⊙) and a shell (Rshell ∼ 9 R⊙) around the companion, which is probably a combination of the matter from the past mass transfer, the mass being lost by the Cepheid owing to wind and pulsations, and a decretion disk around a rapidly rotating secondary. Our study, together with observational properties of pWVir stars, suggests that the majority of them are products of a similar binary evolution interaction. [ABSTRACT FROM AUTHOR]

It is unknown whether or not low-mass stars can form at low metallicity. While theoretical simulations of Population III (Pop III) star formation show that protostellar disks can fragment, it is impossible for those simulations to discern if those fragments survive as low-mass stars. We report the discovery of a low-mass star on a circular orbit with orbital period P = 34.757 ± 0.010 days in the ultra metal-poor (UMP) single-lined spectroscopic binary system 2MASS J18082002–5104378. The secondary star 2MASS J18082002–5104378 B has a mass , placing it near the hydrogen-burning limit for its composition. The 2MASS J18082002–5104378 system is on a thin disk orbit as well, making it the most metal-poor thin disk star system by a considerable margin. The discovery of 2MASS J18082002–5104378 B confirms the existence of low-mass UMP stars and its short orbital period shows that fragmentation in metal-poor protostellar disks can lead to the formation and survival of low-mass stars. We use scaling relations for the typical fragment mass and migration time along with published models of protostellar disks around both UMP and primordial composition stars to explore the formation of low-mass Pop III stars via disk fragmentation. We find evidence that the survival of low-mass secondaries around solar-mass UMP primaries implies the survival of solar-mass secondaries around Pop III primaries with masses . If true, this inference suggests that solar-mass Pop III stars formed via disk fragmentation could survive to the present day. [ABSTRACT FROM AUTHOR]

Based on new observations and improved modeling techniques, we have reanalyzed seven Cepheids in the Large Magellanic Cloud. Improved physical parameters have been determined for the exotic system OGLE LMC-CEP-1718 composed of two first-overtone Cepheids and a completely new model was obtained for the OGLE LMC-CEP-1812 classical Cepheid. This is now the shortest period Cepheid for which the projection factor is measured. The typical accuracy of our dynamical masses and radii determinations is 1%. The radii of the six classical Cepheids follow period–radius relations in the literature. Our very accurate physical parameter measurements allow us to calculate a purely empirical, tight period-mass–radius relation that agrees well with theoretical relations derived from non-canonical models. This empirical relation is a powerful tool to calculate accurate masses for single Cepheids for which precise radii can be obtained from Baade–Wesselink-type analyses. The mass of the type-II Cepheid κ Pav, 0.56 ± 0.08 M⊙, determined using this relation is in a very good agreement with theoretical predictions. We find large differences between the p-factor values derived for the Cepheids in our sample. Evidence is presented that a simple period–p-factor relation shows an intrinsic dispersion, hinting at the relevance of other parameters, such as the masses, radii, and radial velocity variation amplitudes. We also find evidence that the systematic blueshift exhibited by Cepheids is primarily correlated with their gravity. The companion star of the Cepheid in the OGLE LMC-CEP-4506 system has a very similar temperature and luminosity, and is clearly located inside the Cepheid instability strip, yet it is not pulsating. [ABSTRACT FROM AUTHOR]

The recent detection of a binary neutron star merger and the clear evidence of the decay of radioactive material observed in this event have, after 60 years of effort, provided an astrophysical site for the rapid neutron-capture (r-) process which is responsible for the production of the heaviest elements in our universe. However, observations of metal-poor stars with highly enhanced r-process elements have revealed abundance patterns suggesting that multiple sites may be involved. To address this issue, and to advance our understanding of the r-process, we have initiated an extensive search for bright (V < 13.5), very metal-poor ([Fe/H] < −2) stars in the Milky Way halo exhibiting strongly enhanced r-process signatures. This paper presents the first sample collected in the southern hemisphere using the echelle spectrograph on du Pont 2.5 m telescope at Las Campanas Observatory. We have observed and analyzed 107 stars with −3.13 < [Fe/H] < −0.79. Of those, 12 stars are strongly enhanced in heavy r-process elements (r-II), 42 stars show moderate enhancements of heavy r-process material (r-I), and 20 stars exhibit low abundances of the heavy r-process elements and higher abundances of the light r-process elements relative to the heavy ones (limited-r). This search is more successful at finding r-process-enhanced stars compared to previous searches, primarily due to a refined target selection procedure that focuses on red giants. [ABSTRACT FROM AUTHOR]

The study of the chemical abundances of metal-poor stars in dwarf galaxies provides a venue to constrain paradigms of chemical enrichment and galaxy formation. Here we present metallicity and carbon abundance measurements of 100 stars in Sculptor from medium-resolution (R ∼ 2000) spectra taken with the Magellan/Michigan Fiber System mounted on the Magellan-Clay 6.5 m telescope at Las Campanas Observatory. We identify 24 extremely metal-poor star candidates ([Fe/H] < −3.0) and 21 carbon-enhanced metal-poor (CEMP) star candidates. Eight carbon-enhanced stars are classified with at least 2σ confidence, and five are confirmed as such with follow-up R ∼ 6000 observations using the Magellan Echellette Spectrograph on the Magellan-Baade 6.5 m telescope. We measure a CEMP fraction of 36% for stars below [Fe/H] = −3.0, indicating that the prevalence of carbon-enhanced stars in Sculptor is similar to that of the halo (∼43%) after excluding likely CEMP-s and CEMP-r/s stars from our sample. However, we do not detect that any CEMP stars are strongly enhanced in carbon ([C/Fe] > 1.0). The existence of a large number of CEMP stars both in the halo and in Sculptor suggests that some halo CEMP stars may have originated from accreted early analogs of dwarf galaxies. [ABSTRACT FROM AUTHOR]

We present the results of a study of the type II Cepheid (Ppuls = 4.974 days) in the eclipsing binary system OGLE-LMC-T2CEP-098 (Porb = 397.2 days). The Cepheid belongs to the peculiar W Vir group, for which the evolutionary status is virtually unknown. It is the first single-lined system with a pulsating component analyzed using the method developed by Pilecki et al. We show that the presence of a pulsator makes it possible to derive accurate physical parameters of the stars even if radial velocities can be measured for only one of the components. We have used four different methods to limit and estimate the physical parameters, eventually obtaining precise results by combining pulsation theory with the spectroscopic and photometric solutions. The Cepheid radius, mass, and temperature are , , and , respectively, while its companion has a similar size (), but is more massive () and hotter (9500 K). Our best estimate for the p-factor of the Cepheid is . The mass, position on the period–luminosity diagram, and pulsation amplitude indicate that the pulsating component is very similar to the Anomalous Cepheids, although it has a much longer period and is redder in color. The very unusual combination of the components suggest that the system has passed through a mass-transfer phase in its evolution. More complicated internal structure would then explain its peculiarity. [ABSTRACT FROM AUTHOR]

In an effort to measure the masses of planets discovered by the NASA K2 mission, we have conducted precise Doppler observations of five stars with transiting planets. We present the results of a joint analysis of these new data and previously published Doppler data. The first star, an M dwarf known as K2-3 or EPIC 201367065, has three transiting planets (“b,” with radius “c,” and “d,” ). Our analysis leads to the mass constraints: and Mc < 4.2 M⊕ (95% confidence). The mass of planet d is poorly constrained because its orbital period is close to the stellar rotation period, making it difficult to disentangle the planetary signal from spurious Doppler shifts due to stellar activity. The second star, a G dwarf known as K2-19 or EPIC 201505350, has two planets (“b,” 7.7 R⊕; and “c,” 4.9 R⊕) in a 3:2 mean-motion resonance, as well as a shorter-period planet (“d,” 1.1 R⊕). We find Mb = , Mc = and Md < 14.0 M⊕ (95% conf.). The third star, a G dwarf known as K2-24 or EPIC 203771098, hosts two transiting planets (“b,” 5.7 R⊕; and “c,” 7.8 R⊕) with orbital periods in a nearly 2:1 ratio. We find Mb = and Mc = . The fourth star, a G dwarf known as EPIC 204129699, hosts a hot Jupiter for which we measured the mass to be . The fifth star, a G dwarf known as EPIC 205071984, contains three transiting planets (“b,” 5.4 R⊕; “c,” 3.5 R⊕; and “d,” 3.8 R⊕), the outer two of which have a nearly 2:1 period ratio. We find Mb = , Mc < (95% conf.) and Md < 35 M⊕ (95% conf.). [ABSTRACT FROM AUTHOR]

We present a detailed study of the classical Cepheid in the double-lined, highly eccentric eclipsing binary system OGLE-LMC562.05.9009. The Cepheid is a fundamental mode pulsator with a period of 2.988 days. The orbital period of the system is 1550 days. Using spectroscopic data from three 4–8-m telescopes and photometry spanning 22 years, we were able to derive the dynamical masses and radii of both stars with exquisite accuracy. Both stars in the system are very similar in mass, radius, and color, but the companion is a stable, non-pulsating star. The Cepheid is slightly more massive and bigger (M1 = 3.70 ± 0.03 M⊙, R1 = 28.6 ± 0.2 R⊙) than its companion (M2 = 3.60 ± 0.03 M⊙, R2 = 26.6 ± 0.2 R⊙). Within the observational uncertainties both stars have the same effective temperature of 6030 ± 150 K. Evolutionary tracks place both stars inside the classical Cepheid instability strip, but it is likely that future improved temperature estimates will move the stable giant companion just beyond the red edge of the instability strip. Within current observational and theoretical uncertainties, both stars fit on a 205 Myr isochrone arguing for their common age. From our model, we determine a value of the projection factor of p = 1.37 ± 0.07 for the Cepheid in the OGLE-LMC562.05.9009 system. This is the second Cepheid for which we could measure its p-factor with high precision directly from the analysis of an eclipsing binary system, which represents an important contribution toward a better calibration of Baade-Wesselink methods of distance determination for Cepheids. [ABSTRACT FROM AUTHOR]

We present here the first spectroscopic and photometric analysis of the double-lined eclipsing binary containing the classical, first-overtone (FO) Cepheid OGLE-LMC-CEP-2532 (MACHO 81.8997.87). The system has an orbital period of 800 days and the Cepheid is pulsating with a period of 2.035 days. Using spectroscopic data from three high-class telescopes and photometry from three surveys spanning 7500 days, we are able to derive the dynamical masses for both stars with an accuracy better than 3%. This makes the Cepheid in this system one of a few classical Cepheids with an accurate dynamical mass determination (). The companion is probably slightly less massive (), but may have the same mass within errors (). The system has an age of about 185 million years and the Cepheid is in a more advanced evolutionary stage. For the first time precise parameters are derived for both stars in this system. Due to the lack of the secondary eclipse for many years, not much was known about the Cepheid’s companion. In our analysis, we used extra information from the pulsations and the orbital solution from the radial velocity curve. The best model predicts a grazing secondary eclipse shallower than 1 mmag, hence undetectable in the data, about 370 days after the primary eclipse. The dynamical mass obtained here is the most accurate known for a FO Cepheid and will contribute to the solution of the Cepheid mass discrepancy problem. [ABSTRACT FROM AUTHOR]