- ID:
- ivo://CDS.VizieR/J/ApJ/762/26
- Title:
- Most metal-poor stars. II. 190 Galactic halo stars
- Short Name:
- J/ApJ/762/26
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a homogeneous chemical abundance analysis of 16 elements in 190 metal-poor Galactic halo stars (38 program and 152 literature objects). The sample includes 171 stars with [Fe/H]<=-2.5, of which 86 are extremely metal poor, [Fe/H]<=-3.0. Our program stars include 10 new objects with [Fe/H]<=-3.5. We identify a sample of "normal" metal-poor stars and measure the trends between [X/Fe] and [Fe/H], as well as the dispersion about the mean trend for this sample. Using this mean trend, we identify objects that are chemically peculiar relative to "normal" stars at the same metallicity. These chemically unusual stars include CEMP-no objects, one star with high [Si/Fe], another with high [Ba/Sr], and one with unusually low [X/Fe] for all elements heavier than Na. The Sr and Ba abundances indicate that there may be two nucleosynthetic processes at lowest metallicity that are distinct from the main r-process. Finally, for many elements, we find a significant trend between [X/Fe] versus T_eff_, which likely reflects non-LTE and/or three-dimensional effects. Such trends demonstrate that care must be exercised when using abundance measurements in metal-poor stars to constrain chemical evolution and/or nucleosynthesis predictions.
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- ID:
- ivo://CDS.VizieR/J/ApJ/762/27
- Title:
- Most metal-poor stars. III. 86 [Fe/H]<=-3.0 stars
- Short Name:
- J/ApJ/762/27
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We examine the metallicity distribution function (MDF) and fraction of carbon-enhanced metal-poor (CEMP) stars in a sample that includes 86 stars with [Fe/H]<=-3.0, based on high-resolution, high signal-to-noise spectroscopy, of which some 32 objects lie below [Fe/H]=-3.5. After accounting for the completeness function, the "corrected" MDF does not exhibit the sudden drop at [Fe/H]=-3.6 that was found in recent samples of dwarfs and giants from the Hamburg/ESO survey. Rather, the MDF decreases smoothly down to [Fe/H]=-4.1. Similar results are obtained from the "raw" MDF. We find that the fraction of CEMP objects below [Fe/H]=-3.0 is 23%+/-6% and 32%+/-8% when adopting the Beers & Christlieb (2005ARA&A..43..531B) and Aoki et al. (2007, J/ApJ/655/492) CEMP definitions, respectively. The former value is in fair agreement with some previous measurements, which adopt the Beers & Christlieb criterion.
- ID:
- ivo://CDS.VizieR/J/ApJ/749/77
- Title:
- MW halo. III. Statistical chemical abundances
- Short Name:
- J/ApJ/749/77
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We find that the relative contribution of satellite galaxies accreted at high redshift to the stellar population of the Milky Way's smooth halo increases with distance, becoming observable relative to the classical smooth halo about 15 kpc from the Galactic center. In particular, we determine line-of-sight-averaged [Fe/H] and [{alpha}/Fe] in the metal-poor main-sequence turnoff (MPMSTO) population along every Sloan Extension for Galactic Understanding and Exploration (SEGUE) spectroscopic line of sight. Restricting our sample to those lines of sight along which we do not detect elements of cold halo substructure (ECHOS), we compile the largest spectroscopic sample of stars in the smooth component of the halo ever observed in situ beyond 10 kpc. We find significant spatial autocorrelation in [Fe/H] in the MPMSTO population in the distant half of our sample beyond about 15 kpc from the Galactic center. Inside of 15 kpc however, we find no significant spatial autocorrelation in [Fe/H]. At the same time, we perform SEGUE-like observations of N-body simulations of Milky Way analog formation. While we find that halos formed entirely by accreted satellite galaxies provide a poor match to our observations of the halo within 15 kpc of the Galactic center, we do observe spatial autocorrelation in [Fe/H] in the simulations at larger distances. This observation is an example of statistical chemical tagging and indicates that spatial autocorrelation in metallicity is a generic feature of stellar halos formed from accreted satellite galaxies.
- ID:
- ivo://CDS.VizieR/J/ApJ/784/19
- Title:
- Orphan stream high-resolution spectroscopic study
- Short Name:
- J/ApJ/784/19
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the first high-resolution spectroscopic study on the Orphan stream for five stream candidates, observed with the Magellan Inamori Kyocera Echelle spectrograph on the Magellan Clay telescope. The targets were selected from the low-resolution catalog of Casey et al. (2013ApJ...764...39C): three high-probability members, one medium, and one low-probability stream candidate were observed. Our analysis indicates that the low- and medium-probability targets are metal-rich field stars. The remaining three high-probability targets range over ~1 dex in metallicity, and are chemically distinct compared to the other two targets and all standard stars: low [{alpha}/Fe] abundance ratios are observed, and lower limits are ascertained for [Ba/Y], which sit well above the Milky Way trend. These chemical signatures demonstrate that the undiscovered parent system is unequivocally a dwarf spheroidal galaxy, consistent with dynamical constraints inferred from the stream width and arc. As such, we firmly exclude the proposed association between NGC 2419 and the Orphan stream. A wide range in metallicities adds to the similarities between the Orphan stream and Segue 1, although the low [{alpha}/Fe] abundance ratios in the Orphan stream are in tension with the high [{alpha}/Fe] values observed in Segue 1. Open questions remain before Segue 1 could possibly be claimed as the "parent" of the Orphan stream. The parent system could well remain undiscovered in the southern sky.
- ID:
- ivo://CDS.VizieR/J/ApJ/757/164
- Title:
- Oxygen abundances of dwarf stars
- Short Name:
- J/ApJ/757/164
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Oxygen abundances of 67 dwarf stars in the metallicity range -1.6<[Fe/H]<-0.4 are derived from a non-LTE analysis of the 777nm OI triplet lines. These stars have precise atmospheric parameters measured by Nissen and Schuster (Cat. J/A+A/511/L10), who find that they separate into three groups based on their kinematics and {alpha}-element (Mg, Si, Ca, Ti) abundances: thick disk, high-{alpha} halo, and low-{alpha} halo. We find the oxygen abundance trends of thick-disk and high-{alpha} halo stars very similar. The low-{alpha} stars show a larger star-to-star scatter in [O/Fe] at a given [Fe/H] and have systematically lower oxygen abundances compared to the other two groups. Thus, we find the behavior of oxygen abundances in these groups of stars similar to that of the {alpha} elements. We use previously published oxygen abundance data of disk and very metal-poor halo stars to present an overall view (-2.3<[Fe/H]<+0.3) of oxygen abundance trends of stars in the solar neighborhood. Two field halo dwarf stars stand out in their O and Na abundances. Both G53-41 and G150-40 have very low oxygen and very high sodium abundances, which are key signatures of the abundance anomalies observed in globular cluster (GC) stars. Therefore, they are likely field halo stars born in GCs. If true, we estimate that at least 3%+/-2% of the local field metal-poor star population was born in GCs.
- ID:
- ivo://CDS.VizieR/J/PASP/103/973
- Title:
- Photoelectric UBV from HK survey. II.
- Short Name:
- J/PASP/103/973
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Photoelectric UBV photometry is presented for a sample of 165 stars drawn from the extension of the HK objective-prism/interference-filter survey of Beers, Preston and Shectman (1985AJ.....90.2089B) to the northern galactic hemisphere. These results are part of continuing follow-up observations of candidate metal-deficient and horizontal-branch field stars and other interesting stars identified in the survey. A complete set of photometric observations is now available for three of the 25-square-degree survey fields. We estimate that the complete photometric sample of 299 northern HK candidates obtained to date contains on order 90 main-sequences turnoff, subgiant, and giant stars with [Fe/H]<=-1.0, 50 field blue horizontal-branch stars, and 30 stars with the photometric properties of field blue stragglers.
- ID:
- ivo://CDS.VizieR/J/ApJ/813/89
- Title:
- Radial velocities of Milky Way inner halo stars
- Short Name:
- J/ApJ/813/89
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We measure the three components of velocity dispersion, {sigma}_R_, {sigma}_{theta}_, {sigma}_{phi}_, for stars within 6<R<30kpc of the Milky Way using a new radial velocity sample from the MMT telescope. We combine our measurements with previously published data so that we can more finely sample the stellar halo. We use a maximum likelihood statistical method for estimating mean velocities, dispersions, and covariances assuming only that velocities are normally distributed. The alignment of the velocity ellipsoid is consistent with a spherically symmetric gravitational potential. From the spherical Jeans equation, the mass of the Milky Way is M(R<=12kpc)=1.3x10^11^M_{sun}_ with an uncertainty of 40%. We also find a region of discontinuity, 15<~R<~25kpc, where the estimated velocity dispersions and anisotropies diverge from their anticipated values, confirming the break observed by others. We argue that this break in anisotropy is physically explained by coherent stellar velocity structure in the halo, such as the Sgr stream. To significantly improve our understanding of halo kinematics will require combining radial velocities with future Gaia proper motions.
- ID:
- ivo://CDS.VizieR/J/ApJ/795/52
- Title:
- Red giant star sample from SDSS
- Short Name:
- J/ApJ/795/52
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have obtained a sample of ~22000 red giant branch (RGB) stars based on stellar parameters, provided by the ninth data release of the Sloan Digital Sky Survey (Cat. V/139), and the CH(G)/MgH indices, measured from the included spectra. The Galactic rest-frame velocity of V_gsr_ versus longitude for the sample shows the existence of several groups of stars from globular clusters and known streams. Excluding these substructures, a sample of ~16000 RGB stars from the general field is used to investigate the properties of the thick disk, the inner halo, and the outer halo of our Galaxy. The metallicity and rotational velocity distributions are investigated for stars at 0 kpc<|Z|<10 kpc. It is found that the canonical thick disk dominates at 0 kpc<|Z|<2 kpc and its contribution becomes negligible at |Z|>3 kpc. The MWTD is present and overlaps with the inner halo at 1 kpc<|Z|<3 kpc. The inner halo starts at 2 kpc<|Z|<3 kpc and becomes the dominated population for 4 kpc<|Z|<10 kpc. For halo stars with |Z|>5 kpc, bimodal metallicity distributions are found for 20 kpc<|Z|<25 kpc and 35 kpc<RR<45 kpc, which suggests a dual halo, the inner and the outer halo, as reported in Carollo et al. (2007Natur.450.1020C) at low|Z| values. The peak of metallicity for the inner halo is at [Fe/H]~-1.6 and appears to be at [Fe/H]~-2.3 for the outer halo. The transition point from the inner to the outer halo is located at|Z|~20 kpc and RR~35 kpc.
- ID:
- ivo://CDS.VizieR/J/ApJ/781/22
- Title:
- Region I of La Silla QUEST RR Lyrae star survey
- Short Name:
- J/ApJ/781/22
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- A search for RR Lyrae stars (RRLSs) in ~840deg^2^ of the sky in right ascension 150{deg}-210{deg} and declination -10{deg} to + 10{deg} yielded 1013 type ab and 359 type c RRLS. This sample is used to study the density profile of the Galactic halo, halo substructures, and the Oosterhoff type of the halo over distances (d_{sun}_) from ~5 to ~80kpc. The halo is flattened toward the Galactic plane, and its density profile steepens in slope at galactocentric distances greater than ~25kpc. The RRLS in the stellar stream from the Sagittarius dwarf spheroidal (dSph) galaxy match well the model of Law & Majewski for the stars that were stripped 1.3-3.2Gyr ago, but not for the ones stripped 3.2-5.0Gyr ago. Over densities are found at the locations of the Virgo Overdensity and the Virgo Stellar Stream. Within 1{deg} of 1220-1, which Jerjen et al. identify as a halo substructure at d_{sun}_~24kpc, there are four RRLS that are possibly members. Away from substructures, the RRLS are a mixture of Oosterhoff types I and II, but mostly OoI (~73%). The accretion of galaxies resembling in RRLS content the most massive Milky Way satellites (LMC, SMC, For, Sgr) may explain this preponderance of OoI. Six new RRLS and three new anomalous Cepheids were found in the Sextans dSph galaxy.
- ID:
- ivo://CDS.VizieR/J/ApJ/889/177
- Title:
- RGB abundances in the disk, stream & halo of M31
- Short Name:
- J/ApJ/889/177
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We measured [Fe/H] and [{alpha}/Fe] using spectral synthesis of low-resolution stellar spectroscopy for 70 individual red-giant-branch stars across four fields spanning the outer disk, Giant Stellar Stream (GSS), and inner halo of M31. Fields at M31-centric projected distances of 23kpc in the halo, 12kpc in the halo, 22kpc in the GSS, and 26kpc in the outer disk are {alpha}-enhanced, with <[{alpha}/Fe]>=0.43, 0.50, 0.41, and 0.58, respectively. The 23 and 12kpc halo fields are relatively metal-poor, with <[Fe/H]>=-1.54 and -1.30, whereas the 22kpc GSS and 26kpc outer disk fields are relatively metal-rich with <[Fe/H]>=-0.84 and -0.92, respectively. For fields with substructure, we separated the stellar populations into kinematically hot stellar halo components and kinematically cold components. We did not find any evidence of a radial [{alpha}/Fe] gradient along the high surface brightness core of the GSS between ~17 and 22kpc. However, we found tentative suggestions of a negative radial [{alpha}/Fe] gradient in the stellar halo, which may indicate that different progenitor(s) or formation mechanisms contributed to the build up of the inner versus outer halo. Additionally, the [{alpha}/Fe] distribution of the metal-rich ([Fe/H]>-1.5), smooth inner stellar halo (r_proj_<~26kpc) is inconsistent with having formed from the disruption of a progenitor(s) similar to present-day M31 satellite galaxies. The 26kpc outer disk is most likely associated with the extended disk of M31, where its high {alpha}-enhancement provides support for an episode of rapid star formation in M31's disk possibly induced by a major merger.
