- ID:
- ivo://CDS.VizieR/J/ApJ/889/157
- Title:
- Ages of FGK stars considering C and O abundances
- Short Name:
- J/ApJ/889/157
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Precise stellar ages of stars are necessary to study the evolution of the Milky Way. The age determination is significantly affected by C and O abundances of stars due to their contribution to the overall metallicity and opacity. On the basis of C and O abundances derived from high-resolution observations, we determine the ages of 148 FGK-type dwarfs in the solar neighborhood by considering C and O enhancements individually. Our results show that using C and O enhancements individually could affect the age determination of the high-{alpha} population, especially for stars with [O/{alpha}]>0.2dex, making them about 1Gyr younger compared to the results using traditional {alpha}-enhanced models. This results in a steeper slope in the age-[{alpha}/Fe] relation for the high-{alpha} population (changes from 0.0339+/-0.0075 to 0.0436+/-0.0086), indicating a higher formation rate. We find no tight relation between age and [{alpha}/Fe] or [O/Fe] in the high-{alpha} populations. The distribution of space velocity for young {alpha}-rich stars shows that they are more likely characterized to the low-{alpha} populations.
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Search Results
- ID:
- ivo://CDS.VizieR/J/ApJS/253/58
- Title:
- Ages of field stars from white dwarf comp. in Gaia
- Short Name:
- J/ApJS/253/58
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We analyze 4050 wide binary star systems involving a white dwarf (WD) and usually a main-sequence (MS) star, drawn from the large sample assembled by Tian+ (2020, J/ApJS/246/4). Using the modeling code BASE-9, we determine the system's ages, the WD progenitors' zero-age MS masses, the extinction values (AV), and the distance moduli. Discarding the cases with poor age convergences, we obtain ages for 3551 WDs, with a median age precision of {sigma}{tau}/{tau}=20%, and system ages typically in the range of 1-6Gyr. We validated these ages against the very few known clusters and through cross validation of 236 WD-WD binaries. Under the assumption that the components are coeval in a binary system, this provides precise age constraints on the usually low-mass MS companions, mostly inaccessible by any other means.
- ID:
- ivo://CDS.VizieR/J/A+A/348/897
- Title:
- Ages of main-sequence stars
- Short Name:
- J/A+A/348/897
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have determined the age of a sample of nearby main-sequence stars with spectral types B9-K9. We have derived the stellar ages from five different age estimators: the location in the HR diagram compared to theoretical isochrones, the rotational velocity, the strength of chromospheric calcium emission lines, the stellar metallicity, and their space velocity. New calibrations consistent with recent theoretical isochrones are provided for the last four indicators. For hot stars, isochrones are the best indicator, while stellar rotation is best for cool stars. However, many stars require in fact a combination of different methods to properly bracket their actual age. We also discuss the uncertainties involved, in particular those in using isochrones, and we find that these uncertainties are often underestimated in the literature.
- ID:
- ivo://CDS.VizieR/J/other/RMxAA/47.361
- Title:
- AGN activity in isolated SDSS galaxies
- Short Name:
- J/other/RMxAA/47
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We discuss the nature and origin of the nuclear activity observed in a sample of 292 SDSS narrow-emission-line galaxies, considered to have formed and evolved in isolation. All these galaxies are spiral like and show some kind of nuclear activity. The fraction of Narrow Line AGNs (NLAGNs) and Transition type Objects (TOs; a NLAGN with circumnuclear star formation) is relatively high, amounting to 64% of the galaxies. There is a definite trend for the NLAGNs to appear in early-type spirals, while the star forming galaxies and TOs are found in later-type spirals. We verify that the probability for a galaxy to show an AGN characteristic increases with the bulge mass of the galaxy (Torres-Papaqui et al. 2011), and find evidence that this trend is really a by-product of the morphology, suggesting that the AGN phenomenon is intimately connected with the formation process of the galaxies. Consistent with this interpretation, we establish a strong connection between the astration rate -- the efficiency with which the gas is transformed into stars - the AGN phenomenon, and the gravitational binding energy of the galaxies: the higher the binding energy, the higher the astration rate and the higher the probability to find an AGN. The NLAGNs in our sample are consistent with scaled-down or powered-down versions of quasars and Broad Line AGNs.
- ID:
- ivo://CDS.VizieR/J/A+A/601/A10
- Title:
- A grid of MARCS model atmospheres for S stars
- Short Name:
- J/A+A/601/A10
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- S-type stars are late-type giants whose atmospheres are enriched in carbon and s-process elements because of either extrinsic pollution by a binary companion or intrinsic nucleosynthesis and dredge-up on the thermally-pulsing asymptotic giant branch. A grid of MARCS model atmospheres has been computed for S stars, covering the range 2700<=Teff(K)<=4000, 0.50<=C/O<0.99, 0<=logg<=5, [Fe/H]=0., -0.5dex, and [s/Fe]= 0, 1, and 2 dex (where the latter quantity refers to the global overabundance of s-process elements). The MARCS models make use of a new ZrO line list. Synthetic spectra computed from these models are used to derive photometric indices in the Johnson and Geneva systems, as well as TiO and ZrO band strengths. A method is proposed to select the model best matching any given S star, a non-trivial operation since the grid contains more than 3500 models covering a five-dimensional parameter space. The method is based on the comparison between observed and synthetic photometric indices and spectral band strengths, and has been applied on a vast subsample of the Henize sample of S stars. Our results confirm the old claim by Piccirillo (1980MNRAS.190..441P) that ZrO bands in warm S stars (Teff > 3200K) are not caused by the C/O ratio being close to unity, as traditionally believed, but rather by some Zr overabundance. The TiO and ZrO band strengths, combined with V-K and J-K photometric indices, are used to select Teff, C/O, [Fe/H] and [s/Fe]. The Geneva U-B_1 and B_2-V_1 indices (or any equivalent) are good at selecting the gravity. The defining spectral features of dwarf S stars are outlined, but none is found among the Henize S stars. More generally, it is found that, at Teff=3200K, a change of C/O from 0.5 to 0.99 has a strong impact on V-K (2mag). Conversely, a range of 2 mag in V-K corresponds to a 200K shift along the (Teff, V-K) relationship (for a fixed C/O value). Hence, the use of a (Teff, V-K) calibration established for M stars will yield large errors for S stars, so that a specific calibration must be used, as provided in the present paper. Using the atmospheric parameters derived by our method for the sample of Henize S stars, we show that the extrinsic-intrinsic dichotomy among S stars reveals itself very clearly as a bimodal distribution in the effective temperatures. Moreover, the increase of s-process element abundances with increasing C/O ratios and decreasing temperatures is apparent among intrinsic stars, confirming theoretical expectations.
286. A-G star metallicity
- ID:
- ivo://CDS.VizieR/J/MNRAS/320/451
- Title:
- A-G star metallicity
- Short Name:
- J/MNRAS/320/451
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The Edinburgh-Cape Blue Object Survey is an ongoing project to identify and analyse a large sample of hot stars selected initially on the basis of photographic colours (down to a magnitude limit (B~18.0) over the entire high-Galactic-latitude southern sky, and then studied with broad-band UBV photometry and medium-resolution spectroscopy. Due to unavoidable errors in the initial candidate selection, stars that are likely metal-deficient dwarfs and giants of the halo and thick-disc populations are inadvertently included, yet are of interest in their own right. In this paper we discuss a total of 206 candidate metal-deficient dwarfs, subgiants, giants, and horizontal-branch stars with photoelectric colours redder than (B-V)_0_=0.3, and with available spectroscopy. Radial velocities, accurate to ~10-15km/s, are presented for all of these stars. Spectroscopic metallicity estimates for these stars are obtained using a recently recalibrated relation between Ca II K-line strength and (B-V)_0_ colour. The identification of metal-poor stars from this colour-selection technique is remarkably efficient, and competitive with previous survey methods. An additional sample of 186 EC stars with photoelectric colours in the range -0.4<=(B-V)_0_<0.3, photoelectric colours in the range composed primarily of field horizontal-branch stars and other, higher gravity, A- and B-type stars, is also analysed. Estimates of the physical parameters T_eff_, log g and [Fe/H] are obtained for cooler members of this subsample, and a number of candidate RR Lyrae variables are identified.
- ID:
- ivo://CDS.VizieR/J/A+A/618/A16
- Title:
- A(Li) and 6Li/7Li 3D NLTE corrections
- Short Name:
- J/A+A/618/A16
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The purpose of this work is to provide corrections for the lithium abundance, A(Li), and the ^6^Li/^7^Li ratio that can easily be applied to correct 1D LTE lithium abundances in G and F dwarf stars of approximately solar mass and metallicity for three-dimensional (3D) and non-LTE (NLTE) effects. The corrections for A(Li) and ^6^Li/^7^Li are computed using grids of 3D NLTE and 1D LTE synthetic lithium line profiles, generated from 3D hydro-dynamical CO5BOLD and 1D hydrostatic LHD model atmospheres, respectively. For comparative purposes, all calculations are performed for three different line lists representing the LiI 670.8nm spectral region. The 3D NLTE corrections are then approximated by analytical expressions as a function of the stellar parameters (Teff, logg, [Fe/H], vsini, A(Li), ^6^Li/^7^Li). The derived 3D NLTE corrections range between -0.01 and +0.11dex for A(Li), and between -4.9 and -0.4% for the ^6^Li/^7^Li ratio, depending on the adopted stellar parameters. Our findings show a general consistency with recent works on lithium abundance corrections. In the case of the ^6^Li/^7^Li ratio, our corrections are always negative, showing that 1D LTE analysis can significantly overestimate the presence of ^6^Li (up to 4.9 percentage points) in the atmospheres of solar-like dwarf stars. These results emphasize the importance of reliable 3D model atmospheres combined with NLTE line formation for deriving precise isotopic lithium abundances. Although 3D NLTE spectral synthesis implies an extensive computational effort, the results can be made accessible with parametric tools like the ones presented in this work.
- ID:
- ivo://CDS.VizieR/J/ApJ/836/77
- Title:
- A library of high-S/N optical spectra of FGKM stars
- Short Name:
- J/ApJ/836/77
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Classification of stars, by comparing their optical spectra to a few dozen spectral standards, has been a workhorse of observational astronomy for more than a century. Here, we extend this technique by compiling a library of optical spectra of 404 touchstone stars observed with Keck/HIRES by the California Planet Search. The spectra have high resolution (R~60000), high signal-to-noise ratio (S/N~150/pixel), and are registered onto a common wavelength scale. The library stars have properties derived from interferometry, asteroseismology, LTE spectral synthesis, and spectrophotometry. To address a lack of well-characterized late-K dwarfs in the literature, we measure stellar radii and temperatures for 23 nearby K dwarfs, using modeling of the spectral energy distribution and Gaia parallaxes. This library represents a uniform data set spanning the spectral types ~M5-F1 (T_eff_~3000-7000K, R_*_~0.1-16R_{Sun}_). We also present "Empirical SpecMatch" (SpecMatch-Emp), a tool for parameterizing unknown spectra by comparing them against our spectral library. For FGKM stars, SpecMatch-Emp achieves accuracies of 100K in effective temperature (T_eff_), 15% in stellar radius (R_*_), and 0.09dex in metallicity ([Fe/H]). Because the code relies on empirical spectra it performs particularly well for stars ~K4 and later, which are challenging to model with existing spectral synthesizers, reaching accuracies of 70K in T_eff_, 10% in R_*_, and 0.12dex in [Fe/H]. We also validate the performance of SpecMatch-Emp, finding it to be robust at lower spectral resolution and S/N, enabling the characterization of faint late-type stars. Both the library and stellar characterization code are publicly available.
- ID:
- ivo://CDS.VizieR/J/A+A/604/A53
- Title:
- ALLSMOG final data release. A new APEX CO survey
- Short Name:
- J/A+A/604/A53
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the final data release of the APEX low-redshift legacy survey for molecular gas (ALLSMOG), comprising CO(2-1) emission line observations of 88 nearby, low-mass (10^8.5^<M_*_[M_{sun}_]<10^10^) star-forming galaxies carried out with the 230GHz APEX-1 receiver on the APEX telescope. The main goal of ALLSMOG is to probe the molecular gas content of more typical and lower stellar mass galaxies than have been studied by previous CO surveys. We also present IRAM 30m observations of the CO(1-0) and CO(2-1) emission lines in nine galaxies aimed at increasing the M_*_<10^9^M_{sun}_ sample size.In this paper we describe the observations, data reduction and analysis methods and we present the final CO spectra together with archival HI 21cm line observations for the entire sample of 97 galaxies. At the sensitivity limit of ALLSMOG, we register a total CO detection rate of 47%. Galaxies with higher M_*_, SFR, nebular extinction (A_V_), gas-phase metallicity (O/H), and HI gas mass have systematically higher CO detection rates.In particular, the parameter according to which CO detections and non-detections show the strongest statistical differences is the gas-phase metallicity, for any of the five metallicity calibrations examined in this work. We investigate scaling relations between the CO(1-0) line luminosity (L'_CO(1-0)_) and galaxy-averaged properties using ALLSMOG and a sub-sample of COLD GASS for a total of 185 sources that probe the local main sequence (MS) of star-forming galaxies and its +/-0.3dex intrinsic scatter from M_*_=10^8.5^M_{sun}_ to M_*_=10^11^M_{sun}_. L'_CO(1-0)_ is most strongly correlated with the SFR, but the correlation with M_*_ is closer to linear and almost comparably tight. The relation between L'_CO(1-0)_ and metallicity is the steepest one, although deeper CO observations of galaxies with A_V_<0.5mag may reveal an as much steep correlation with A_V_. Our results suggest that star-forming galaxies across more than two orders of magnitude in M_*_ obey similar scaling relations between CO luminosity and the galaxy properties examined in this work. Besides SFR, the CO luminosity is likely most fundamentally linked to M_*_, although we note that stellar mass alone cannot explain all of the variation in CO emission observed as a function of O/H and M_HI_.
- ID:
- ivo://CDS.VizieR/J/A+A/636/A67
- Title:
- ALMA maps of 6 sources of star forming regions
- Short Name:
- J/A+A/636/A67
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- As a building block for amino acids, formamide (NH_2_CHO) is an important molecule in astrobiology and astrochemistry, but its formation path in the interstellar medium is not understood well. We aim to find empirical evidence to support the chemical relationships of formamide to HNCO and H_2_CO. We examine high angular resolution (~0.2") Atacama Large Millimeter/submillimeter Array (ALMA) maps of six sources in three high-mass star-forming regions and compare the spatial extent, integrated emission peak position, and velocity structure of HNCO and H_2_CO line emission with that of NH_2_CHO by using moment maps. Through spectral modeling, we compare the abundances of these three species. In these sources, the emission peak separation and velocity dispersion of formamide emission is most often similar to HNCO emission, while the velocity structure is generally just as similar to H_2_CO and HNCO (within errors). From the spectral modeling, we see that the abundances between all three of our focus species are correlated, and the relationship between NH_2_CHO and HNCO reproduces the previously demonstrated abundance relationship. In this first interferometric study, which compares two potential parent species to NH_2_CHO, we find that all moment maps for HNCO are more similar to NH_2_CHO than H_2_CO in one of our six sources (G24 A1). For the other five sources, the relationship between NH_2_CHO, HNCO, and H_2_CO is unclear as the different moment maps for each source are not consistently more similar to one species as opposed to the other.