- ID:
- ivo://CDS.VizieR/J/AJ/117/2308
- Title:
- Spectroscopy of hot stars in the halo
- Short Name:
- J/AJ/117/2308
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We discuss a spectroscopic and photometric technique that enables the identification and classification of field horizontal-branch (FHB) and other A-type stars, even from relatively low signal-to-noise ratio medium-resolution spectra. This technique makes use of broadband UBV colors predicted from model atmosphere calculations and Balmer line profiles and Ca II K equivalent widths determined from synthetic spectra to estimate the physical parameters T_eff_, log(g), and [Fe/H] for stars in the effective temperature range 6000 - 10,000 K.
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- ID:
- ivo://CDS.VizieR/J/AJ/90/2089
- Title:
- Stars of very low metal abundance. I
- Short Name:
- J/AJ/90/2089
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- An objective-prism and a narrowband interference filter have been used in combination on the Curtis Schmidt telescope at CTIO to conduct a search for extremely metal-poor stars. The filter restricts the bandpass to the region around the H and K lines of CaII; the metal-poor candidates are those stars with weak or absent H and K. Slit spectra (1 A resolution) of some 450 candidates are used to measure equivalent widths of the CaII K line and Balmer features. A calibration of the K-line strength for giants in the metal-poor globular cluster NGC 6397 and field dwarfs and giants of known metallicity is used to identify survey candidates with metal abundance [Fe/H]<=-2.0; a list of 134 such stars is presented. UBV photometry is used to distinguish between two groups of metal-poor stars: a group with objects near the main-sequence turnoff, on the subgiant branch, and on the giant branch; and a second group with objects on the red horizontal branch (RHB) and asymptotic giant branch (AGB). The RHB-AGB stars exhibit the well-known weak G-band effect; three CH-strong objects are identified among the subgiants.
- ID:
- ivo://CDS.VizieR/J/ApJ/714/663
- Title:
- Stellar density map of the Milky Way
- Short Name:
- J/ApJ/714/663
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We map the stellar structure of the Galactic thick disk and halo by applying color-magnitude diagram (CMD) fitting to photometric data from the Sloan Extension for Galactic Understanding and Exploration (SEGUE) survey. The SEGUE imaging scans allow, for the first time, a comprehensive analysis of Milky Way structure at both high and low latitudes using uniform Sloan Digital Sky Survey photometry. Incorporating photometry of all relevant stars simultaneously, CMD fitting bypasses the need to choose single tracer populations. Using old stellar populations of differing metallicities as templates, we obtain a sparse three-dimensional map of the stellar mass distribution at |Z|>1kpc. Fitting a smooth Milky Way model comprising exponential thin and thick disks and an axisymmetric power-law halo allows us to constrain the structural parameters of the thick disk and halo. The thick-disk scale height and length are well constrained at 0.75+/-0.07kpc and 4.1+/-0.4kpc, respectively.
- ID:
- ivo://CDS.VizieR/J/A+A/657/A41
- Title:
- Stellar halo of NGC 5128 (Centaurus A)
- Short Name:
- J/A+A/657/A41
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- The extended stellar halos of galaxies contain important clues for investigating their assembly history and evolution. We investigate the resolved stellar content and the extended halo of NGC 5128 as a function of galactocentric distance, and trace the halo outward to its currently detectable limits. We used Hubble Space Telescope images obtained with the WFPC2, ACS, and WFC3 cameras equipped with F606W and F814W filters to resolve individual red giant branch (RGB) stars in 28 independent pointings across the halo of NGC 5128. The stellar halo analysis for 14 of these pointings is presented here for the first time. Star counts from deep VI color-magnitude diagrams reaching at least 1.5mag below the tip of the RGB are used to derive the surface density distribution of the halo. The contamination by Milky Way stars is assessed with a new control field, with models, and by combining optical and near-IR photometry. We present a new calibration of the WFC3 F606W + F814W photometry to the ground-based VI photometric system. The photometry shows that the stellar halo of NGC 5128 is dominated by old RGB stars that are present in all fields. The V-band surface brightness of fields changes from 23 to 32mag/arcsec^2^ between the innermost field only 8.3kpc from the galaxy center to our outermost halo fields, which are located 140 kpc away from the center along the major axis and 92 kpc along the minor axis. Within the inner ~30kpc, we also find evidence for a 2-3Gyr old population traced by asymptotic giant branch stars that are brighter than the tip of the RGB. This population contributes only up to 10% in total stellar mass if it is 2Gyr old, but a larger fraction of 30-40% is required if its age is 3Gyr. The stellar surface density profile is well fit by a classic r^1/4^ curve or a simple power-law form ~r^-3.1^ over the full radial range, with no obvious break in the slope, but with large field-to-field scatter. The ellipticity measured from integrated-light photometry in the inner parts, e=(b/a)=0.77, flattens to e=0.54+/-0.02 beyond 30kpc. Considering the flattening of the outer halo, the projection of the elliptical isophote on the semimajor axis for our most distant field reaches nearly 30 effective radii.
- ID:
- ivo://CDS.VizieR/J/ApJ/784/170
- Title:
- The SEGUE K giant survey. II. Distances of 6036 stars
- Short Name:
- J/ApJ/784/170
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present an online catalog of distance determinations for 6036 K giants, most of which are members of the Milky Way's stellar halo. Their medium-resolution spectra from the Sloan Digital Sky Survey/Sloan Extension for Galactic Understanding and Exploration are used to derive metallicities and rough gravity estimates, along with radial velocities. Distance moduli are derived from a comparison of each star's apparent magnitude with the absolute magnitude of empirically calibrated color-luminosity fiducials, at the observed (g-r)_0_ color and spectroscopic [Fe/H]. We employ a probabilistic approach that makes it straightforward to properly propagate the errors in metallicities, magnitudes, and colors into distance uncertainties. We also fold in prior information about the giant-branch luminosity function and the different metallicity distributions of the SEGUE K-giant targeting sub-categories. We show that the metallicity prior plays a small role in the distance estimates, but that neglecting the luminosity prior could lead to a systematic distance modulus bias of up to 0.25mag, compared to the case of using the luminosity prior. We find a median distance precision of 16%, with distance estimates most precise for the least metal-poor stars near the tip of the red giant branch. The precision and accuracy of our distance estimates are validated with observations of globular and open clusters. The stars in our catalog are up to 125kpc from the Galactic center, with 283 stars beyond 50kpc, forming the largest available spectroscopic sample of distant tracers in the Galactic halo.
- ID:
- ivo://CDS.VizieR/J/PASP/102/1392
- Title:
- UBV photometry from HK survey. I
- Short Name:
- J/PASP/102/1392
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Photoelectric photometry is presented for a sample of 139 halo stars drawn an extension of the HK (Ca II H & K lines) objective-prism survey of Beers, Preston and Shectmann (1988ApJS...67..461) to the Northern Galactic hemisphere. The candidates for which photometry is reported here were selected to span a wide range of types, but are dominated by stars classified as type AB, A, or "metal-poor" (MP).
- ID:
- ivo://CDS.VizieR/J/AJ/87/1515
- Title:
- UBV photometry of AB stars in halo
- Short Name:
- J/AJ/87/1515
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The results of photoelectric UBV photometry of 234 AB stars in the southern galactic halo are presented. The stars were chosen from the Curtis Schmidt objective-prism survey of Preston and Shectman and comprise a sample of halo objects selected without kinematic bias. The majority of the stars lie in the region of the (U-B, B-V) plane occupied by field and globular cluster blue horizontal-branch stars.
- ID:
- ivo://CDS.VizieR/J/ApJS/76/1001
- Title:
- UBV photometry of HK survey stars
- Short Name:
- J/ApJS/76/1001
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Photoelectric UBV observations of 1782 stars are used to explore properties of the HK objective-prism survey conducted by Beers, Preston, and Shectman with the Curtis-Schmidt telescope of the Cerro Tololo Inter-American Observatory. We construct an intrinsic (U-B)0 versus (B-V)0 relation for blue horizontal branch stars and use it to derive estimates of E(B-V) color excesses in 50 survey fields. Comparison of these with the reddening maps of Burstein and Heiles indicates good agreement except in low-latitude fields toward the Galactic bulge. We describe qualitative features of the stellar content of the HK survey by use of unreddened two-color diagrams and indicate how the photometric data may prove useful in several applications. Finally, we use a halo density model to estimate the variation of completeness with apparent magnitude, B, in our catalog of field horizontal branch star candidates.
- ID:
- ivo://CDS.VizieR/J/ApJ/756/23
- Title:
- V-band photometric follow-up of RR Lyrae from SEKBO
- Short Name:
- J/ApJ/756/23
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We investigate the steepening of the power-law slope beyond the Galactocentric radius of R~45kpc in the RR Lyrae (RRL) space density distribution found by Keller et al. (2008, Cat. J/ApJ/678/851). They identified 2016 RRL candidates derived from the analysis of archival observations of the Southern Edgeworth-Kuiper Belt Object (SEKBO) survey. Our aim is to verify the completeness of RRLs and ultimately the space density distribution function given by Keller et al. We followed up on a subset of 137 candidates with a range of magnitudes (V~14-20) using the Faulkes Telescope (FT) database and confirmed 57 candidates as real RRLs. A cross-match between SEKBO RRL survey and Sloan Digital Sky Survey Data Release-7 (SDSS DR-7) revealed 272 RRL candidates in common. Applying the color selection criteria proposed by Ivezi\'c et al. (2005AJ....129.1096I) resulted in 193 likely RRLs. The completeness as a function of magnitude was calculated empirically from the combined set of SEKBO RRL candidates from current FT data, SDSS cross-matched data, and the Prior et al. catalog (2009, Cat. J/ApJ/691/306). This resulted in a spatial density distribution characterized by two power laws with a break radius R within a range between 45kpc and 50kpc, similar to the results of Keller et al. We find the power-law slopes for the inner halo as n_inner_=-2.78+/-0.02 and for the outer halo as n_outer_=-5.0+/-0.2.
- ID:
- ivo://CDS.VizieR/J/ApJ/588/824
- Title:
- Velocities of blue stars near (l,b)=(198,-27)
- Short Name:
- J/ApJ/588/824
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present evidence for a ring of stars in the plane of the Milky Way, extending at least from l=180{deg} to 227{deg} with turnoff magnitude g~19.5; the ring could encircle the Galaxy. We infer that the low Galactic latitude structure is at a fairly constant distance of R=18+/-2kpc from the Galactic center above the Galactic plane and has R=20+/-2kpc in the region sampled below the Galactic plane. The evidence includes 500 Sloan Digital Sky Survey spectroscopic radial velocities of stars within 30{deg} of the plane. The velocity dispersion of the stars associated with this structure is found to be 27km/s at (l, b)=(198, -27), 22km/s at (l, b)=(225, 28), 30km/s at (l, b)=(188, 24), and 30km/s at (l, b)=(182, 27). The structure rotates in the same prograde direction as the Galactic disk stars but with a circular velocity of 110+/-25km/s. The narrow measured velocity dispersion is inconsistent with power-law spheroid or thick-disk populations.
