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- ID:
- ivo://CDS.VizieR/J/AJ/155/256
- Title:
- Spectroscopic validation of RAVE metal-poor stars
- Short Name:
- J/AJ/155/256
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present results from a medium-resolution (R~2000) spectroscopic follow-up campaign of 1694 bright (V<13.5), very metal-poor star candidates from the RAdial Velocity Experiment (RAVE). Initial selection of the low-metallicity targets was based on the stellar parameters published in RAVE Data Releases 4 (Cat. III/272) and 5 (Cat. III/279). Follow up was accomplished with the Gemini-N and Gemini-S, the ESO/NTT, the KPNO/Mayall, and the SOAR telescopes. The wavelength coverage for most of the observed spectra allows for the determination of carbon and {alpha}-element abundances, which are crucial for considering the nature and frequency of the carbon-enhanced metal-poor (CEMP) stars in this sample. We find that 88% of the observed stars have [Fe/H]=< -1.0, 61% have [Fe/H]=< -2.0, and 3% have [Fe/H]=< -3.0 (with four stars at [Fe/H]=< -3.5). There are 306 CEMP star candidates in this sample, and we identify 169 CEMP Group I, 131 CEMP Group II, and 6 CEMP Group III stars from the A(C) versus [Fe/H] diagram. Inspection of the [{alpha}/C] abundance ratios reveals that five of the CEMP Group II stars can be classified as "mono-enriched second-generation" stars. Gaia DR1 matches were found for 734 stars, and we show that transverse velocities can be used as a confirmatory selection criteria for low-metallicity candidates. Selected stars from our validated list are being followed-up with high-resolution spectroscopy to reveal their full chemical-abundance patterns for further studies.
- ID:
- ivo://CDS.VizieR/J/ApJ/870/122
- Title:
- Spectroscopy of low-metallicity star candidates
- Short Name:
- J/ApJ/870/122
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present results from an observing campaign to identify low-metallicity stars in the Best & Brightest Survey. From medium-resolution (R~1200-2000) spectroscopy of 857 candidates, we estimate the stellar atmospheric parameters (T_eff_, logg, and [Fe/H]), as well as carbon and {alpha}-element abundances. We find that 69% of the observed stars have [Fe/H]<=-1.0, 39% have [Fe/H]<=-2.0, and 2% have [Fe/H]<=-3.0. There are also 133 carbon-enhanced metal-poor (CEMP) stars in this sample, with 97 CEMP Group I and 36 CEMP Group II stars identified in the A(C) versus [Fe/H] diagram. A subset of the confirmed low-metallicity stars were followed-up with high-resolution spectroscopy, as part of the R-process Alliance, with the goal of identifying new highly and moderately r-process-enhanced stars. Comparison between the stellar atmospheric parameters estimated in this work and from high-resolution spectroscopy exhibit good agreement, confirming our expectation that medium-resolution observing campaigns are an effective way of selecting interesting stars for further, more targeted, efforts.
- ID:
- ivo://CDS.VizieR/J/A+A/626/A92
- Title:
- Spitzer/IRS analysis of the 30-micron sources
- Short Name:
- J/A+A/626/A92
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present an analysis and comparison of the 30{mu}m dust features seen in the Spitzer Space Telescope spectra of 207 carbon-rich asymptotic giant branch (AGB) stars, post-AGB objects, and planetary nebulae (PNe) located in the Milky Way, the Magellanic Clouds (MCs), or the Sagittarius dwarf spheroidal galaxy (Sgr dSph), which are characterised by different average metallicities. We investigated whether the formation of the 30{mu}m feature carrier may be a function of the metallicity. Through this study we expect to better understand the late stages of stellar evolution of carbon-rich stars in these galaxies. Our analysis uses the "Manchester method" as a basis for estimating the temperature of dust for the carbon-rich AGB stars and the PNe in our sample. For post-AGB objects we changed the wavelength ranges used for temperature estimation, because of the presence of the 21{mu}m feature on the short wavelength edge of the 30{mu}m feature. We used a black-body function with a single temperature deduced from the Manchester method or its modification to approximate the continuum under the 30{mu}m feature. We find that the strength of the 30{mu}m feature increases until dust temperature drops below 400K. Below this temperature, the large loss of mass and probably the self-absorption effect reduces the strength of the feature. During the post-AGB phase, when the intense mass-loss has terminated, the optical depth of the circumstellar envelope is smaller, and the 30{um}m feature becomes visible again, showing variety of values for post-AGB objects and PNe, and being comparable with the strengths of AGB stars. In addition, the AGB stars and post-AGB objects show similar values of central wavelengths - usually between 28.5 and 29.5{mu}m. However, in case of PNe the shift of the central wavelength towards longer wavelengths is visible. The normalised median profiles for AGB stars look uniformly for various ranges of dust temperature, and different galaxies. We analysed the profiles of post-AGB objects and PNe only within one dust temperature range (below 200K), and they were also similar in different galaxies. In the spectra of 17 PNe and five post-AGB objects we found the broad 16-24{mu}m feature. Two objects among the PNe group are the new detections: SMP LMC 51, and SMP LMC 79, whereas in the case of post-AGBs the new detections are: IRAS 05370-7019, IRAS 05537-7015, and IRAS 21546+4721. In addition, in the spectra of nine PNe we found the new detections of 16-18{mu}m feature. We also find that the Galactic post-AGB object IRAS 11339-6004 has a 21{mu}m emission. Finally, we have produced online catalogues of photometric data and Spitzer IRS spectra for all objects that show the 30{mu}m feature. These resources are available online for use by the community. The most important conclusion of our work is the fact that the formation of the 30{mu}m feature is affected by metallicity. Specifically that, as opposed to more metal-poor samples of AGB stars in the MCs, the feature is seen at lower mass-loss rates, higher temperatures, and has seen to be more prominent in Galactic carbon stars. The averaged feature (profile) in the AGB, post-AGB objects, and PNe seems unaffected by metallicity at least between a fifth and solar metallicity, but in the case of PNe it is shifted to significantly longer wavelengths.
- ID:
- ivo://CDS.VizieR/J/ApJ/826/44
- Title:
- Spitzer/IRS obs. of Magellanic carbon stars
- Short Name:
- J/ApJ/826/44
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The Infrared Spectrograph on the Spitzer Space Telescope observed 184 carbon stars in the Magellanic Clouds. This sample reveals that the dust-production rate (DPR) from carbon stars generally increases with the pulsation period of the star. The composition of the dust grains follows two condensation sequences, with more SiC condensing before amorphous carbon in metal-rich stars, and the order reversed in metal-poor stars. MgS dust condenses in optically thicker dust shells, and its condensation is delayed in more metal-poor stars. Metal-poor carbon stars also tend to have stronger absorption from C_2_H_2_ at 7.5{mu}m. The relation between DPR and pulsation period shows significant apparent scatter, which results from the initial mass of the star, with more massive stars occupying a sequence parallel to lower-mass stars, but shifted to longer periods. Accounting for differences in the mass distribution between the carbon stars observed in the Small and Large Magellanic Clouds reveals a hint of a subtle decrease in the DPR at lower metallicities, but it is not statistically significant. The most deeply embedded carbon stars have lower variability amplitudes and show SiC in absorption. In some cases they have bluer colors at shorter wavelengths, suggesting that the central star is becoming visible. These deeply embedded stars may be evolving off of the asymptotic giant branch and/or they may have non-spherical dust geometries.
- ID:
- ivo://CDS.VizieR/J/A+A/574/A129
- Title:
- The First CEMP star in the Sculptor dSph
- Short Name:
- J/A+A/574/A129
- Date:
- 04 Feb 2022 11:54:02
- Publisher:
- CDS
- Description:
- he origin of carbon-enhanced metal-poor (CEMP) stars and their possible connection with the chemical elements produced by the first stellar generation is still highly debated. In contrast to the Galactic halo, not many CEMP stars have been found in the dwarf spheroidal galaxies around the Milky Way. Here we present detailed abundances from ESO VLT/UVES high-resolution spectroscopy for ET0097, the first CEMP star found in the Sculptor dwarf spheroidal, which is one of the best studied dwarf galaxies in the Local Group. This star has [Fe/H]=-2.03+/-0.10, [C/Fe]=0.51+/-0.10 and [N/Fe]=1.18+/-0.20, which is the first nitrogen measurement in this galaxy. The traditional definition of CEMP stars is [C/Fe]>=0.70, but taking into account that this luminous red giant branch star has undergone mixing, it was intrinsically less nitrogen enhanced and more carbon-rich when it was formed, and so it falls under the definition of CEMP stars, as proposed by Aoki et al. (2007ApJ...655..492A, Cat. J/ApJ/655/492) to account for this effect.
- ID:
- ivo://CDS.VizieR/J/A+A/621/A50
- Title:
- TX Psc ALMA CO(2-1) images
- Short Name:
- J/A+A/621/A50
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We observed the carbon-rich asymptotic giant branch (AGB) star TX Piscium (TX Psc) with ALMA in CO(2-1) emission to investigate the circumstellar envelope (CSE) and mass-loss history of this object. Previous observations with Herschel in the far infrared have shown a ring-like structure in dust emission (2011A&A...532A.135J). Our molecular gas observations of the CO(2-1) emission line cover this structure with significantly higher spatial resolution to investigate its origin.
- ID:
- ivo://CDS.VizieR/J/AJ/159/21
- Title:
- V and Rc light curves of medium-bright PPNe
- Short Name:
- J/AJ/159/21
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present 10 years of new photometric monitoring of the light variability of five evolved stars with strong mid-infrared emission from surrounding dust. Three are known carbon-rich proto-planetary nebulae (PPNe) with F-G spectral types; the nature of the other two was previously unknown. For the three PPNe, we determine or refine the pulsation periods of IRAS04296+3429 (71 days), 06530-0213 (80 days), and 23304+6147 (84 days). A secondary period was found for each, with a period ratio P_2_/P_1_ of 0.9. The light variations are small, 0.1-0.2mag. These are similar to values found in other PPNe. The other two are found to be giant stars. IRAS09296+1159 pulsates with a period of only 47 days but reaches pulsational light variations of 0.5mag. Supplemental spectroscopy reveals the spectrum of a CH carbon star. IRAS08359-1644 is a G1III star that does not display pulsational variability; rather, it shows nonperiodic decreases of brightness of up to 0.5mag over this 10 year interval. These drops in brightness are reminiscent of the light curves of R Corona Borealis variables, but with much smaller decreases in brightness and are likely due to transient dust obscuration. Its spectral energy distribution is very similar to that of the unusual oxygen-rich giant star HDE233517, which possesses mid-infrared hydrocarbon emission features. These two non-PPNe turn out to be members of the rare group of giant stars with large mid-infrared excesses due to dust, objects which presumably have interesting evolutionary histories.
- ID:
- ivo://CDS.VizieR/J/ApJ/709/1042
- Title:
- Variability in C-rich proto-PNe.
- Short Name:
- J/ApJ/709/1042
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have carried out long-term (14 years) V and R photometric monitoring of 12 carbon-rich proto-planetary nebulae. The light and color curves display variability in all of them. The light curves are complex and suggest multiple periods, changing periods, and/or changing amplitudes, which are attributed to pulsation. A dominant period has been determined for each and found to be in the range of ~150 days for the coolest (G8) to 35-40 days for the warmest (F3). A clear, linear inverse relationship has been found in the sample between the pulsation period and the effective temperature and also an inverse relationship between the amplitude of light variation and the effective temperature. These are consistent with the expectation for a pulsating post-asymptotic giant branch (post-AGB) star evolving toward higher temperature at constant luminosity. The published spectral energy distributions and mid-infrared images show these objects to have cool (200K), detached dust shells and published models imply that intensive mass loss ended 400-2000 years ago.
- ID:
- ivo://CDS.VizieR/J/ApJ/766/116
- Title:
- Variability in proto-PNe. II.
- Short Name:
- J/ApJ/766/116
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have carried out a detailed observational study of the light, color, and velocity variations of two bright, carbon-rich proto-planetary nebulae, IRAS 22223+4327 and 22272+5435. The light curves are based upon our observations from 1994 to 2011, together with published data by Arkhipova and collaborators. They each display four significant periods, with primary periods for IRAS 22223+4327 and 22272+5435 being 90 and 132 days, respectively. For each of them, the ratio of secondary to primary period is 0.95, a value much different from that found in Cepheids, but which may be characteristic of post-asymptotic giant branch (AGB) stars. Fewer significant periods are found in the smaller radial velocity data sets, but they agree with those of the light curves. The color curves generally mimic the light curves, with the objects reddest when faintest. A comparison in seasons when there exist contemporaneous light, color, and velocity curves reveals that the light and color curves are in phase, while the radial velocity curves are ~0.25 P out of phase with the light curves. Thus they differ from what is seen in Cepheids, in which the radial velocity curve is 0.50 P out of phase with the light curve. Comparison of the observed periods and amplitudes with those of post-AGB pulsation models shows poor agreement, especially for the periods, which are much longer than predicted. These observational data, particularly the contemporaneous light, color, and velocity curves, provide an excellent benchmark for new pulsation models of cool stars in the post-AGB, proto-planetary nebula phase.
