- ID:
- ivo://CDS.VizieR/J/AJ/155/138
- Title:
- WOCS.LXXVI.Velocity & abundances in NGC2506
- Short Name:
- J/AJ/155/138
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- HYDRA spectra of 287 stars in the field of NGC 2506 from the turnoff through the giant branch are analyzed. With previous data, 22 are identified as probable binaries; 90 more are classified as potential non-members. Spectroscopic analyses of ~60 red giants and slowly rotating turnoff stars using line equivalent widths and a neural network approach lead to [Fe/H]=-0.27+/-0.07 (s.d.) and [Fe/H]=-0.27+/-0.06 (s.d.), respectively. Li abundances are derived for 145 probable single-star members, 44 being upper limits. Among turnoff stars outside the Li-dip, A(Li)=3.04+/-0.16 (s.d.), with no trend with color, luminosity, or rotation speed. Evolving from the turnoff across the subgiant branch, there is a well-delineated decline to A(Li)~1.25 at the giant branch base, coupled with the rotational spindown from between ~20 and 70 km/s to less than 20 km/s for stars entering the subgiant branch and beyond. A(Li) remains effectively constant from the giant branch base to the red giant clump level. A new member above the clump redefines the path of the first-ascent red giant branch; its Li is 0.6 dex below the first-ascent red giants. With one exception, all post-He-flash stars have upper limits to A(Li), at or below the level of the brightest first-ascent red giant. The patterns are in excellent qualitative agreement with the model predictions for low/intermediate-mass stars which undergo rotation-induced mixing at the turnoff and subgiant branch, first dredge-up, and thermohaline mixing beyond the red giant bump.
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- ID:
- ivo://CDS.VizieR/J/AJ/162/92
- Title:
- 126 X-rays sources around the cepheid {eta} Aql
- Short Name:
- J/AJ/162/92
- Date:
- 14 Mar 2022 07:02:20
- Publisher:
- CDS
- Description:
- X-ray bursts have recently been discovered in the Cepheids {delta}Cep and {beta}Dor modulated by the pulsation cycle. We have obtained an observation of the Cepheid {eta}Aql with the XMM-Newton satellite at the phase of maximum radius; the phase at which there is a burst of X-rays in {delta}Cep. No X-rays were seen from the Cepheid {eta}Aql at this phase, and the implications for Cepheid upper atmospheres are discussed. We have also used the combination of X-ray sources, as well as Gaia and 2MASS data, to search for a possible grouping around the young intermediate mass Cepheid. No indication of such a group was found.
- ID:
- ivo://CDS.VizieR/J/ApJ/749/177
- Title:
- Yellow and red supergiants in the LMC
- Short Name:
- J/ApJ/749/177
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Due to their transitionary nature, yellow supergiants (YSGs) provide a critical challenge for evolutionary modeling. Previous studies within M31 and the Small Magellanic Cloud show that the Geneva evolutionary models do a poor job at predicting the lifetimes of these short-lived stars. Here, we extend this study to the Large Magellanic Cloud (LMC) while also investigating the galaxy's red supergiant (RSG) content. This task is complicated by contamination by Galactic foreground stars that color and magnitude criteria alone cannot weed out. Therefore, we use proper-motions and the LMC's large systemic radial velocity (~278km/s) to separate out these foreground dwarfs. After observing nearly 2000 stars, we identified 317 probable YSGs, 6 possible YSGs, and 505 probable RSGs. Foreground contamination of our YSG sample was ~80%, while that of the RSG sample was only 3%. By placing the YSGs on the Hertzsprung-Russell diagram and comparing them against the evolutionary tracks, we find that new Geneva evolutionary models do an exemplary job at predicting both the locations and the lifetimes of these transitory objects.
- ID:
- ivo://CDS.VizieR/J/ApJ/719/1784
- Title:
- Yellow supergiants in the SMC
- Short Name:
- J/ApJ/719/1784
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The yellow supergiant content of nearby galaxies provides a critical test of massive star evolutionary theory. While these stars are the brightest in a galaxy, they are difficult to identify because a large number of foreground Milky Way stars have similar colors and magnitudes. We previously conducted a census of yellow supergiants within M31 and found that the evolutionary tracks predict a yellow supergiant duration an order of magnitude longer than we observed. Here we turn our attention to the Small Magellanic Cloud (SMC), where the metallicity is 10x lower than that of M31, which is important as metallicity strongly affects massive star evolution. The SMC's large radial velocity (~160km/s) allows us to separate members from foreground stars. Observations of ~500 candidates yielded 176 near-certain SMC supergiants, 16 possible SMC supergiants, along with 306 foreground stars, and provide good relative numbers of yellow supergiants down to 12M_{sun}_. Of the 176 near-certain SMC supergiants, the kinematics predicted by the Besancon model of the Milky Way suggest a foreground contamination of <=4%. After placing the SMC supergiants on the Hertzsprung-Russell diagram (HRD) and comparing our results to the Geneva evolutionary tracks, we find results similar to those of the M31 study: while the locations of the stars on the HRD match the locations of evolutionary tracks well, the models overpredict the yellow supergiant lifetime by a factor of 10. Uncertainties about the mass-loss rates on the main sequence thus cannot be the primary problem with the models.
- ID:
- ivo://CDS.VizieR/J/ApJ/832/121
- Title:
- 4-yr RV survey of red giant in eclipsing binaries
- Short Name:
- J/ApJ/832/121
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Given the potential of ensemble asteroseismology for understanding fundamental properties of large numbers of stars, it is critical to determine the accuracy of the scaling relations on which these measurements are based. From several powerful validation techniques, all indications so far show that stellar radius estimates from the asteroseismic scaling relations are accurate to within a few percent. Eclipsing binary systems hosting at least one star with detectable solar-like oscillations constitute the ideal test objects for validating asteroseismic radius and mass inferences. By combining radial velocity (RV) measurements and photometric time series of eclipses, it is possible to determine the masses and radii of each component of a double-lined spectroscopic binary. We report the results of a four-year RV survey performed with the echelle spectrometer of the Astrophysical Research Consortium's 3.5m telescope and the APOGEE spectrometer at Apache Point Observatory. We compare the masses and radii of 10 red giants (RGs) obtained by combining radial velocities and eclipse photometry with the estimates from the asteroseismic scaling relations. We find that the asteroseismic scaling relations overestimate RG radii by about 5% on average and masses by about 15% for stars at various stages of RG evolution. Systematic overestimation of mass leads to underestimation of stellar age, which can have important implications for ensemble asteroseismology used for Galactic studies. As part of a second objective, where asteroseismology is used for understanding binary systems, we confirm that oscillations of RGs in close binaries can be suppressed enough to be undetectable, a hypothesis that was proposed in a previous work.
- ID:
- ivo://CDS.VizieR/J/ApJ/799/53
- Title:
- YSO/MS star models in the central molecular zone
- Short Name:
- J/ApJ/799/53
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In contrast to most other galaxies, star formation rates in the Milky Way can be estimated directly from young stellar objects (YSOs). In the central molecular zone the star formation rate calculated from the number of YSOs with 24{mu}m emission is up to an order of magnitude higher than the value estimated from methods based on diffuse emission (such as free-free emission). Whether this effect is real or whether it indicates problems with either or both star formation rate measures is not currently known. In this paper, we investigate whether estimates based on YSOs could be heavily contaminated by more evolved objects such as main-sequence stars. We present radiative transfer models of YSOs and of main-sequence stars in a constant ambient medium which show that the main-sequence objects can indeed mimic YSOs at 24{mu}m. However, we show that in some cases the main-sequence models can be marginally resolved at 24{mu}m, whereas the YSO models are always unresolved. Based on the fraction of resolved MIPS 24{mu}m sources in the sample of YSOs previously used to compute the star formation rate, we estimate the fraction of misclassified "YSOs" to be at least 63%, which suggests that the star formation rate previously determined from YSOs is likely to be at least a factor of three too high.
- ID:
- ivo://CDS.VizieR/J/AJ/145/107
- Title:
- Zr, Ba, La, and Eu abundances in 19 open clusters
- Short Name:
- J/AJ/145/107
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present an analysis of the s-process elements Zr, Ba, and La and the r-process element Eu in a sample of 50 stars in 19 open clusters. Stellar abundances of each element are based on measures of a minimum of two lines per species via both equivalent width and spectrum synthesis techniques. We investigate cluster mean neutron-capture abundance trends as a function of cluster age and location in the Milky Way disk and compare them to results found in other studies in the literature. We find a statistically significant trend of increasing cluster [Ba/Fe] as a function of decreasing cluster age, in agreement with recent findings for other open cluster samples, supporting the increased importance of low-mass asymptotic giant branch stars to the generation of s-process elements. However, the other s-process elements, [La/Fe] and [Zr/Fe], do not show similar dependences, in contrast to theoretical expectations and the limited observational data from other studies. Conversely, cluster [Eu/Fe] ratios show a slight increase with increasing cluster age, although with marginal statistical significance. Ratios of [s/r]-process abundances, [Ba/Eu] and [La/Eu], however, show more clearly the increasing efficiency of s-process relative to r-process enrichment in open cluster chemical evolution, with significant increases among younger clusters. Last, cluster neutron-capture element abundances appear to be independent of Galactocentric distance. We conclude that a homogeneous analysis of a larger sample of open clusters is needed to resolve the apparent discrepant conclusions between different studies regarding s-process element abundance trends with age to better inform models of galactic chemical evolution.
