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321. Maser and infrared studies of oxygen-rich AGB stars
ID:
ivo://CDS.VizieR/J/ApJ/794/81
Title:
Maser and infrared studies of oxygen-rich AGB stars
Short Name:
J/ApJ/794/81
Date:
21 Oct 2021
Publisher:
CDS
Description:
We explored an efficient method to identify evolved stars with oxygen-rich envelopes in the late asymptotic giant branch (AGB) or post-AGB phase of stellar evolution, which include a rare class of objects - the "water fountains (WF)." Our method considers the OH and H_2_O maser spectra, the near-infrared Q-parameters (these are color indices accounting for the effect of extinction), and far-infrared AKARI colors. Here we first present the results of a new survey on OH and H_2_O masers. There were 108 color-selected objects: 53 of them were observed in the three OH maser lines (1612, 1665, and 1667 MHz), with 24 detections (16 new for 1612 MHz); and 106 of them were observed in the H_2_O maser line (22 GHz), with 24 detections (12 new). We identify a new potential WF source, IRAS 19356+0754, with large velocity coverages of both OH and H_2_O maser emission. In addition, several objects with high-velocity OH maser emission are reported for the first time. The Q-parameters as well as the infrared [09]-[18] and [18]-[65] AKARI colors of the surveyed objects are then calculated. We suggest that these infrared properties are effective in isolating aspherical from spherical objects, but the morphology may not necessarily be related to the evolutionary status. Nonetheless, by considering altogether the maser and infrared properties, the efficiency of identifying oxygen-rich late/post-AGB stars could be improved.
322. Mass and age of red giant branch stars
ID:
ivo://CDS.VizieR/J/MNRAS/475/3633
Title:
Mass and age of red giant branch stars
Short Name:
J/MNRAS/475/3633
Date:
21 Oct 2021
Publisher:
CDS
Description:
Obtaining accurate and precise masses and ages for large numbers of giant stars is of great importance for unraveling the assemblage history of the Galaxy. In this paper, we estimate masses and ages of 6940 red giant branch (RGB) stars with asteroseismic parameters deduced from Kepler photometry and stellar atmospheric parameters derived from LAMOST spectra. The typical uncertainties of mass is a few per cent, and that of age is ~20 per cent. The sample stars reveal two separate sequences in the age-[{alpha}/Fe] relation - a high-{alpha} sequence with stars older than ~8Gyr and a low-{alpha} sequence composed of stars with ages ranging from younger than 1Gyr to older than 11Gyr. We further investigate the feasibility of deducing ages and masses directly from LAMOST spectra with a machine learning method based on kernel based principal component analysis, taking a sub-sample of these RGB stars as a training data set. We demonstrate that ages thus derived achieve an accuracy of ~24 per cent. We also explored the feasibility of estimating ages and masses based on the spectroscopically measured carbon and nitrogen abundances. The results are quite satisfactory and significantly improved compared to the previous studies.
323. Masses and ages of red giants
ID:
ivo://CDS.VizieR/J/MNRAS/456/3655
Title:
Masses and ages of red giants
Short Name:
J/MNRAS/456/3655
Date:
21 Oct 2021
Publisher:
CDS
Description:
We show that the masses of red giant stars can be well predicted from their photospheric carbon and nitrogen abundances, in conjunction with their spectroscopic stellar labels logg, Teff, and [Fe/H]. This is qualitatively expected from mass-dependent post-main-sequence evolution. We here establish an empirical relation between these quantities by drawing on 1475 red giants with asteroseismic mass estimates from Kepler that also have spectroscopic labels from Apache Point Observatory Galactic Evolution Experiment (APOGEE) DR12. We assess the accuracy of our model, and find that it predicts stellar masses with fractional rms errors of about 14 percent (typically 0.2M_{sun}_). From these masses, we derive ages with rms errors of 40 percent. This empirical model allows us for the first time to make age determinations (in the range 1-13Gyr) for vast numbers of giant stars across the Galaxy. We apply our model to ~52000 stars in APOGEE DR12, for which no direct mass and age information was previously available. We find that these estimates highlight the vertical age structure of the Milky Way disc, and that the relation of age with [{alpha}/M] and metallicity is broadly consistent with established expectations based on detailed studies of the solar neighbourhood.
324. Massive stars in APOGEE2 Survey. III.
ID:
ivo://CDS.VizieR/J/ApJS/247/17
Title:
Massive stars in APOGEE2 Survey. III.
Short Name:
J/ApJS/247/17
Date:
21 Oct 2021
Publisher:
CDS
Description:
We have applied the semi-empirical spectral analysis, developed by the Sloan Digital Sky Survey (SDSS)-IV/Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE2) Massive Star Team, to a large sample of new O- and B-type stars identified along the Sagittarius spiral arm, in the direction of the southern star clusters NGC3603 and NGC3576. We obtained H-band spectra for 265 point sources, using the APOGEE2-S spectrograph at the du Pont Telescope at the Las Campanas Observatory. We analyzed the associated spectral features deriving spectral types, as well as the massive star distribution along the line of sight. From a total of 265 science targets, 95 are classified as mid- to late-O-type stars (for which only 10 O-type stars are previously known in the literature), 38 are found to be early- to mid-B-type stars, and 32 are classified as either yellow or blue supergiants, completing a total of 165 massive stars.
325. Massive stars in two giant molecular clouds
ID:
ivo://CDS.VizieR/J/A+A/569/A20
Title:
Massive stars in two giant molecular clouds
Short Name:
J/A+A/569/A20
Date:
21 Oct 2021
Publisher:
CDS
Description:
Young massive stars and stellar clusters continuously form in the Galactic disk, generating new HII regions within their natal giant molecular clouds and subsequently enriching the interstellar medium via their winds and supernovae. Massive stars are among the brightest infrared stars in such regions; their identification permits the characterisation of the star formation history of the associated cloud as well as constraining the location of stellar aggregates and hence their occurrence as a function of global environment. We present a stellar spectroscopic survey in the direction of the giant molecular cloud G23.3-0.3. This complex is located at a distance of ~4-5kpc, and consists of several HII regions and supernova remnants. We discovered 11 Of_K_^+^ stars, one candidate luminous blue variable, several OB stars, and candidate red supergiants. Stars with K-band extinction from ~1.3-1.9mag appear to be associated with the GMC G23.3-0.3; O and B-types satisfying this criterion have spectrophotometric distances consistent with that of the giant molecular cloud. Combining near-IR spectroscopic and photometric data allowed us to characterize the multiple sites of star formation within it. The O-type stars have masses from ~25-45M_{sun}_, and ages of 5-8Myr. Two new red supergiants were detected with interstellar extinction typical of the cloud; along with the two RSGs within the cluster GLIMPSE9, they trace an older burst with an age of 20-30Myr. Massive stars were also detected in the core of three supernova remnants - W41, G22.7-0.2, and G22.7583-0.4917. A large population of massive stars appears associated with the GMC G23.3-0.3, with the properties inferred for them indicative of an extended history of stars formation.
326. Massive star variability in M31 from iPTF
ID:
ivo://CDS.VizieR/J/ApJ/893/11
Title:
Massive star variability in M31 from iPTF
Short Name:
J/ApJ/893/11
Date:
21 Oct 2021
Publisher:
CDS
Description:
Using data from the (intermediate) Palomar Transient Factory (iPTF), we characterize the time variability of ~500 massive stars in M31. Our sample is those stars that are spectrally typed by Massey and collaborators, including Luminous Blue Variables, Wolf-Rayets, and warm and cool supergiants. We use the high-cadence, long-baseline (~5yr) data from the iPTF survey, coupled with data-processing tools that model complex features in the light curves. We find widespread photometric (R-band) variability in the upper Hertzsprung Russell diagram (or CMD) with an increasing prevalence of variability with later spectral types. Red stars (V-I>1.5) exhibit larger amplitude fluctuations than their bluer counterparts. We extract a characteristic variability timescale, t_ch_, via wavelet transformations that are sensitive to both continuous and localized fluctuations. Cool supergiants are characterized by longer timescales (>100 days) than the hotter stars. The latter have typical timescales of tens of days but cover a wider range, from our resolution limit of a few days to longer than 100 days. Using a 60 night block of data straddling two nights with a cadence of around 2 minutes, we extracted t_ch_ in the range 0.1-10 days with amplitudes of a few percent for 13 stars. Though there is broad agreement between the observed variability characteristics in the different parts of the upper CMD with theoretical predictions, detailed comparison requires models with a more comprehensive treatment of the various physical processes operating in these stars, such as pulsation, subsurface convection, and the effect of binary companions.
327. Mass-loss return from LMC evolved stars. VI.
ID:
ivo://CDS.VizieR/J/ApJ/753/71
Title:
Mass-loss return from LMC evolved stars. VI.
Short Name:
J/ApJ/753/71
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present results from the first application of the Grid of Red Supergiant and Asymptotic Giant Branch ModelS (GRAMS) model grid to the entire evolved stellar population of the Large Magellanic Cloud (LMC). GRAMS is a pre-computed grid of 80843 radiative transfer models of evolved stars and circumstellar dust shells composed of either silicate or carbonaceous dust. We fit GRAMS models to ~30000 asymptotic giant branch (AGB) and red supergiant (RSG) stars in the LMC, using 12 bands of photometry from the optical to the mid-infrared. Our published data set consists of thousands of evolved stars with individually determined evolutionary parameters such as luminosity and mass-loss rate. The GRAMS grid has a greater than 80% accuracy rate discriminating between oxygen- and carbon-rich chemistry.
328. 2MASS view of Sgr dSph. VII. Kinematics
ID:
ivo://CDS.VizieR/J/ApJ/756/74
Title:
2MASS view of Sgr dSph. VII. Kinematics
Short Name:
J/ApJ/756/74
Date:
21 Oct 2021
Publisher:
CDS
Description:
We have assembled a large-area spectroscopic survey of giant stars in the Sagittarius (Sgr) dwarf galaxy core. Using medium resolution (R~15000), multifiber spectroscopy we have measured velocities of these stars, which extend up to 12{deg} from the galaxy's center (3.7 core radii or 0.4 times the King limiting radius). From these high-quality spectra we identify 1310 Sgr members out of 2296 stars surveyed, distributed across 24 different fields across the Sgr core. Additional slit spectra were obtained of stars bridging from the Sgr core to its trailing tail. Our systematic, large-area sample shows no evidence for significant rotation, a result at odds with the ~20km/s rotation required as an explanation for the bifurcation seen in the Sgr tidal stream; the observed small (<=4km/s) velocity trend primarily along the major axis is consistent with models of the projected motion of an extended body on the sky with no need for intrinsic rotation. The Sgr core is found to have a flat velocity dispersion (except for a kinematically colder center point) across its surveyed extent and into its tidal tails, a property that matches the velocity dispersion profiles measured for other Milky Way dwarf spheroidal (dSph) galaxies. We comment on the possible significance of this observed kinematical similarity for the dynamical state of the other classical Milky Way dSphs in light of the fact that Sgr is clearly a strongly tidally disrupted system.
329. Metallicities of RGB stars in dwarf galaxies
ID:
ivo://CDS.VizieR/J/ApJ/779/102
Title:
Metallicities of RGB stars in dwarf galaxies
Short Name:
J/ApJ/779/102
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present spectroscopic metallicities of individual stars in seven gas-rich dwarf irregular galaxies (dIrrs), and we show that dIrrs obey the same mass-metallicity relation as the dwarf spheroidal (dSph) satellites of both the Milky Way and M31: Z_{star}_{propto}M_{star}_^0.30+/-0.02^. The uniformity of the relation is in contradiction to previous estimates of metallicity based on photometry. This relationship is roughly continuous with the stellar mass-stellar metallicity relation for galaxies as massive as M_*_=10^12^M_{sun}_. Although the average metallicities of dwarf galaxies depend only on stellar mass, the shapes of their metallicity distributions depend on galaxy type. The metallicity distributions of dIrrs resemble simple, leaky box chemical evolution models, whereas dSphs require an additional parameter, such as gas accretion, to explain the shapes of their metallicity distributions. Furthermore, the metallicity distributions of the more luminous dSphs have sharp, metal-rich cut-offs that are consistent with the sudden truncation of star formation due to ram pressure stripping.
330. Metallicity of bulge clump giants in Baade's window
ID:
ivo://CDS.VizieR/J/A+A/534/A80
Title:
Metallicity of bulge clump giants in Baade's window
Short Name:
J/A+A/534/A80
Date:
21 Oct 2021
Publisher:
CDS
Description:
We seek to constrain the formation of the Galactic bulge by means of analysing the detailed chemical composition of a large sample of red clump stars in Baade's window. These stars were selected to minimise the contamination by other Galactic components, so they are good tracers of the bulge metallicity distribution in Baade's window, at least for stars more metal-rich than ~-1.5. We used an automatic procedure to measure [Fe/H] in a sample of 219 bulge red clump stars from R=20000 resolution spectra obtained with FLAMES/GIRAFFE at the VLT. The analysis was performed differentially with respect to the metal-rich local reference star MuLeo. For a subsample of 162 stars, we also derived [Mg/H] from spectral synthesis around the MgI triplet at 6319{AA}. The Fe and Mg metallicity distributions are both asymmetric with median values of +0.16 and +0.21, respectively. They show only a small proportion of stars at low metallicities, extending down to [Fe/H]=-1.1 or [Mg/H]=-0.7 The iron distribution is clearly bimodal, as revealed both by a deconvolution (from observational errors) and a Gaussian decomposition. The decomposition of the observed Fe and Mg metallicity distributions into Gaussian components yields two populations of equal sizes (50% each): a metal-poor component centred on [Fe/H]=-0.30 and [Mg/H]=-0.06 with a large dispersion and a narrow metal-rich component centred on [Fe/H]=+0.32 and [Mg/H]=+0.35. The metal-poor component shows high [Mg/Fe] ratios (around 0.3), while stars in the metal-rich component are found to have near solar ratios. Kinematical differences between the two components have also been found: the metal-poor component shows kinematics compatible with an old spheroid, while the metal-rich component is consistent with a population supporting a bar. In view of their chemical and kinematical properties, we suggest different formation scenarii for the two populations: a rapid formation time scale as an old spheroid for the metal-poor component (old bulge) and for the metal-rich component, a formation on a longer time scale driven by the evolution of the bar (pseudo-bulge).

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