Field Blue Horizontal Branch Stars Research Articles

Search for stellar spots in field blue horizontal-branch stars

Context. Blue horizontal-branch stars are Population II objects which are burning helium in their core and possess a hydrogen-burning shell and radiative envelope. Because of their low rotational velocities, diffusion has been predicted to work in their atmospheres. In many respects, blue horizontal-branch stars closely resemble the magnetic chemically peculiar stars of the upper main sequence, which show photometric variability caused by abundance spots on their surfaces. These spots are thought to be caused by diffusion and the presence of a stable magnetic field. However, the latter does not seem to be axiomatic. Aims. We searched for rotationally induced variability in 30 well-established bright field blue horizontal-branch stars in the solar neighbourhood and searched the literature for magnetic fields measurements of our targets. Methods. We employed archival photometric time series data from the All Sky Automated Survey (ASAS), All-Sky Automated Survey for Supernovae (ASAS-SN), and Wide Angle Search for Planets (SuperWASP) surveys. The data were carefully reduced and processed, and a time series analysis was applied using several different techniques. We also synthesized existing photometric and spectroscopic data of magnetic chemically peculiar stars in order to study possible different surface characteristics producing lower amplitudes. Results. In the accuracy limit of the employed data, no significant variability signals were found in our sample stars. The resulting upper limits for variability are given. Conclusions. We conclude that either no stellar surface spots are present in field blue horizontal-branch stars, or their characteristics (contrast, total area, and involved elements) are not sufficient to produce amplitudes larger than a few millimagnitudes in the optical wavelength region. New detailed models taking into account the elemental abundance pattern of blue horizontal-branch stars are needed to synthesize light curves for a comparison with our results.

Detailed abundance analysis of five field blue horizontal-branch stars

Detailed abundance analysis of five field blue horizontal-branch stars

CHRONOGRAPHY OF THE MILKY WAY’S HALO SYSTEM WITH FIELD BLUE HORIZONTAL-BRANCH STARS

In a pioneering effort, Preston et al. reported that the colors of blue horizontal-branch (BHB) stars in the halo of the Galaxy shift with distance, from regions near the Galactic center to about 12 kpc away, and interpreted this as a correlated variation in the ages of halo stars, from older to younger, spanning a range of a few Gyrs. We have applied this approach to a sample of some 4700 spectroscopically confirmed BHB stars selected from the Sloan Digital Sky Survey to produce the first "chronographic map" of the halo of the Galaxy. We demonstrate that the mean de-reddened g$-$r color, <(g$-$r)o>, increases outward in the Galaxy from $-$0.22 to $-$0.08 (over a color window spanning [$-$0.3:0.0]) from regions close to the Galactic center to ~40 kpc, independent of the metallicity of the stars. Models of the expected shift in the color of the field BHB stars based on modern stellar evolutionary codes confirm that this color gradient can be associated with an age difference of roughly 2-2.5 Gyrs, with the oldest stars concentrated in the central ~15 kpc of the Galaxy. Within this central region, the age difference spans a mean color range of about 0.05 mag (~0.8 Gyrs). Furthermore, we show that chronographic maps can be used to identify individual substructures, such as the Sagittarius Stream, and overdensities in the direction of Virgo and Monoceros, based on the observed contrast in their mean BHB colors with respect to the foreground/background field population.

TRACING THE METAL-POOR M31 STELLAR HALO WITH BLUE HORIZONTAL BRANCH STARS

We have analyzed new HST/ACS and HST/WFC3 imaging in F475W and F814W of two previously-unobserved fields along the M31 minor axis to confirm our previous constraints on the shape of M31's inner stellar halo. Both of these new datasets reach a depth of at least F814W$<$27 and clearly detect the blue horizontal branch (BHB) of the field as a distinct feature of the color-magnitude diagram. We measure the density of BHB stars and the ratio of BHB to red giant branch stars in each field using identical techniques to our previous work. We find excellent agreement with our previous measurement of a power-law for the 2-D projected surface density with an index of 2.6$^{+0.3}_{-0.2}$ outside of 3 kpc, which flattens to $\alpha <$1.2 inside of 3 kpc. Our findings confirm our previous suggestion that the field BHB stars in M31 are part of the halo population. However, the total halo profile is now known to differ from this BHB profile, which suggests that we have isolated the metal-poor component. This component appears to have an unbroken power-law profile from 3-150 kpc but accounts for only about half of the total halo stellar mass. Discrepancies between the BHB density profile and other measurements of the inner halo are therefore likely due to the different profile of the metal-rich halo component, which is not only steeper than the profile of the met al-poor component, but also has a larger core radius. These profile differences also help to explain the large ratio of BHB/RGB stars in our observations.

Exploring the total Galactic extinction with SDSS BHB stars

Aims: We used 12,530 photometrically-selected blue horizontal branch (BHB) stars from the Sloan Digital Sky Survey (SDSS) to estimate the total extinction of the Milky Way at the high Galactic latitudes, $R_V$ and $A_V$ in each line of sight. Methods: A Bayesian method was developed to estimate the reddening values in the given lines of sight. Based on the most likely values of reddening in multiple colors, we were able to derive the values of $R_V$ and $A_V$. Results: We selected 94 zero-reddened BHB stars from seven globular clusters as the template. The reddening in the four SDSS colors for the northern Galactic cap were estimated by comparing the field BHB stars with the template stars. The accuracy of this estimation is around 0.01\,mag for most lines of sight. We also obtained $<R_V>$ to be around 2.40$\pm1.05$ and $A_V$ map within an uncertainty of 0.1\,mag. The results, including reddening values in the four SDSS colors, $A_V$, and $R_V$ in each line of sight, are released on line. In this work, we employ an up-to-date parallel technique on GPU card to overcome time-consuming computations. We plan to release online the C++ CUDA code used for this analysis. Conclusions: The extinction map derived from BHB stars is highly consistent with that from Schlegel, Finkbeiner & Davis(1998). The derived $R_V$ is around 2.40$\pm1.05$. The contamination probably makes the $R_V$ be larger.

The radial velocity dispersion profile of the Galactic halo: constraining the density profile of the dark halo of the Milky Way

We have compiled a new sample of 240 halo objects with accurate distance and radial velocity measurements, including globular clusters, satellite galaxies, field blue horizontal branch stars and red giant stars from the Spaghetti survey. The new data lead to a significant increase in the number of known objects for Galactocentric radii beyond 50 kpc, which allows a reliable determination of the radial velocity dispersion profile out to very large distances. The radial velocity dispersion shows an almost constant value of 120 km/s out to 30 kpc and then continuously declines down to 50 km/s at about 120 kpc. This fall-off puts important constraints on the density profile and total mass of the dark matter halo of the Milky Way. For a constant velocity anisotropy, the isothermal profile is ruled out, while both a dark halo following a truncated flat model of mass $1.2^{+1.8}_{-0.5}\times 10^{12}$ M_sun and an NFW profile of mass $0.8^{+1.2}_{-0.2}\times 10^{12}$ M_sun and c=18 are consistent with the data. The significant increase in the number of tracers combined with the large extent of the region probed by these has allowed a more precise determination of the Milky Way mass in comparison to previous works. We also show how different assumptions for the velocity anisotropy affect the performance of the mass models.

The Century Survey Galactic Halo Project. II. Global Properties and the Luminosity Function of Field Blue Horizontal Branch Stars

We discuss a 175 deg2 spectroscopic survey for blue horizontal branch (BHB) stars in the Galactic halo. We use the Two Micron All Sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS) to select BHB candidates, and we find that the 2MASS and SDSS color selection is 38% and 50% efficient, respectively, for BHB stars. Our samples include one likely runaway B7 star 6 kpc below the Galactic plane. The global properties of the BHB samples are consistent with membership in the halo population: the median metallicity is [Fe/H] = -1.7, the velocity dispersion is 108 km s-1, and the mean Galactic rotation of the BHB stars 3 kpc < |z| < 15 kpc is -4 ± 30 km s-1. We discuss the theoretical basis of the Preston, Shectman, and Beers MV-color relation for BHB stars and conclude that the intrinsic shape of the BHB MV-color relation results from the physics of stars on the horizontal branch. We calculate the luminosity function for the field BHB star samples using the maximum likelihood method of Efstathiou and coworkers, which is unbiased by density variations. The field BHB luminosity function exhibits a steep rise at bright luminosities, a peak between 0.8 < MV < 1.0, and a tail at faint luminosities. We compare the field BHB luminosity functions with the luminosity functions derived from 16 different globular cluster BHBs. Kolmogorov-Smirnov tests suggest that field BHB stars and BHB stars in globular clusters share a common distribution of luminosities, with the exception of globular clusters with extended BHBs.

Hot Horizontal‐Branch Stars: The Ubiquitous Nature of the “Jump” in Stromgrenu, Low Gravities, and the Role of Radiative Levitation of Metals

A "jump" in the blue horizontal-branch (HB) distribution in the (V, u-y) color-magnitude diagram has recently been detected in the globular cluster (GC) M13 (NGC 6205) by Grundahl and coworkers. Such an effect is morphologically best characterized as a discontinuity in the (u, u-y) locus, with stars in the range 11,500 K ≲ Teff ≲ 20,000 K deviating systematically from (in the sense of appearing brighter and/or hotter than) canonical zero-age HB models. In this article, we present Strömgren u, y photometry of 14 GCs obtained with three different telescopes (ESO Danish, Nordic Optical Telescope, and the Hubble Space Telescope) and demonstrate that the jump in Strömgren u is present in every GC whose HB extends beyond Teff ≳ 11,500 K, irrespective of metallicity, mixing history on the red giant branch (RGB), or any known parameter characterizing our sample of GCs. We thus suggest that the u jump is a ubiquitous feature, intrinsic to all HB stars hotter than Teff ≃ 11,500 K. We draw a parallel between the ubiquitous nature of the u jump and the well-known problem of low measured gravities among blue HB stars in GCs and in the field. We note that the "gravity jump" occurs over the same temperature range as the u jump and also that it occurs in every metal-poor cluster for which gravities have been determined—again irrespective of metallicity, mixing history on the RGB, or any known parameter characterizing the surveyed GCs. Furthermore, we demonstrate that the u jump and the gravity jump are connected on a star-by-star basis. We thus suggest that the two most likely are different manifestations of one and the same physical phenomenon. We present an interpretative framework which may be capable of simultaneously accounting for both the u jump and the gravity jump. Reviewing spectroscopic data for several field blue HB stars, as well as two blue HB stars in NGC 6752, we find evidence that radiative levitation of elements heavier than carbon and nitrogen takes place at Teff ≳ 11,500 K, dramatically enhancing the abundances of such heavy elements in the atmospheres of blue HB stars in the "critical" temperature region. We argue that model atmospheres which take diffusion effects into account are badly needed and will likely lead to better overall agreement between canonical evolutionary theory and the observations for these stars.

Kinematics of the system of blue horizontal branch field stars of the outer Galactic halo

Kinematics of the system of blue horizontal branch field stars of the outer Galactic halo

PHOTOELECTRIC UBV PHOTOMETRY OF NORTHERN STARS FROM THE HK OBJECTIVE-PRISM SURVEY. II.

ABSTRACT 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 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. The complete photometric sample of 299 northern HK candidates obtained to date contains on order 90 main-sequence turnoff, subgiant, and giant stars with (Fe/H) of less than or equal to -1.0, 50 field blue horizontal-branch stars, and 30 stars with the photometric properties of field-blue stragglers.

Detection of a galactic color gradient for blue horizontal-branch stars of the halo field and implications for the halo age and density distributions

From analysis of a large sample of field blue horizontal-branch stars we find that the mean unreddened color, &lt;B-V&gt;<SUB>w</SUB>, in the color window -0.02 &lt;(B-V)<SUB>0</SUB> &lt; +0.18 increases outward in the Galaxy by ∼0.025 on 2 kpc &lt; R &lt; 12 kpc. On the basis of several tests of the sample, we conclude that this result is due neither to errors in reddening estimates nor to contamination of the sample by stars of the young disk or by the population described by Lance (1988b). <P />Calibration of the horizontal-branch (HB) parameter B/(B + R) with two theoretical, age-dependent isochrone parameters and an empirical correlation between &lt;B-V&gt;<SUB>w</SUB> and B/(B + R) suggest that the mean age of the field horizontal branch decreases outward in the halo systematically by a few Gyr. HB models appropriate for standard red giant evolution lead to a similar conclusion, but HB theory also permits an interpretation of the color gradient as a consequence of a systematic outward decrease in HB core mass at constant age, for which there is no plausible reason. <P />We calculate space densities of blue horizontal-branch (BHB) stars from our sample and recalculate space densities for RR Lyrae stars from published data. The RR Lyrae densities are well represented by a spheroidal power-law model in which flattening decreases outward. The power-law exponent for the RR Lyrae stars is n = -3.2±0.1. BHB stars in a more restricted distance range yield n = -3.5, as would be expected from the sign of the color gradient. <P />Predicted densities of BHB and RR Lyrae stars in the solar neighborhood, their ratio (6.5/1), and the abundance distribution and horizontal-branch morphologies of globular clusters are used to estimate the local density of red HB stars and the total local HB number density, 42 kpc<SUP>-3</SUP>. <P />Total HB populations for 44 globular clusters, derived from published photometry and the use of King models, establish that the visual luminosity of a globular cluster is closely proportional to the number of HB stars that it contains. The ratio is L<SUB>v</SUB>/N<SUB>HB</SUB> = 540±40 uncorrected for incompleteness of the counts that would decrease it, or possible radial stratification of HB stars in clusters that would increase it. <P />From the local field HB number density, L<SUB>v</SUB>/N<SUB>HB</SUB>, and an assumed M/L<SUB>v</SUB> = 2.5, we estimate the local halo mass density to be 5.7 × 10<SUP>4</SUP> M<SUB>sun</SUB> kpc<SUP>-3</SUP>, a value within the range estimated by other methods. The local observed density of metal-poor RR Lyrae stars unaccountably exceeds the number predicted by our model by a factor of at least 2.

Blue horizontal branch field stars in the galactic halo - observations versus kinematic models

Une analyse cinematique est effectuee sur un echantillon de 185 etoiles de la branche horizontale bleue situees dans 4 champs du halo galactique. Les dispersions des vitesses observees dans les 4 champs sont comparees avec les predictions de modeles dynamiques du halo et avec un modele phenomenologique. Des scenarios de formation possibles pour le halo galactique en relation avec l'un des modeles dynamiques sont discutes

Stromgren and BV photometry of potential halo blue horizontal branch field stars

Strömgren four-colour and broadband BV photoelectric photometry has been obtained for a sample of potential halo blue horizontal branch stars in two high galactic latitude fields. The large majority of the stars observed are classified as blue horizontal branch stars on the basis of two different surface gravity indicators. Measurements of Ca K-line equivalent widths from medium-dispersion spectra of the stars confirm that most are Population II objects. No metal-rich A stars were found beyond a few kpc from the galactic disc in the study of faint blue stars.

Radial velocities of blue horizontal branch field stars in the outer Galactic halo

On a identifie un echantillon d'etoiles bleues lointaines de la branche horizontale. Trente d'entre elles ont des distances galactocentriques de 15 a 40 kpc. Les vitesses radiales ont ete obtenues avec une grande precision (∼ 20 km s −1 )

AB stars in the southern galactic halo. II - Spectroscopy and radial velocities

view Abstract Citations (95) References (49) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS AB stars in the southern galactic halo. II. Spectroscopy and radial velocities. Pier, J. R. Abstract A spectroscopic study of over two hundred high latitude stars of spectral types A and B which compromise a sample of in situ halo field stars selected without kinematic bias is discussed. Results are presented and discussed in detail for the radial velocities and spectral line measurements of the interstellar K lines, weak metallic lines, Ca II line equivalent widths, and hydrogen lines. Stellar Ca II K line strengths are found to correlate quite well with overall metallicity for all cases except Am stars and high luminosity stars. The K line equivalent widths for all but about five percent of the sample AB stars are normal for metal-poor Population II stars. A few AB stars have near solar metallicities and relatively high radial velocities, but no such stars are found beyond 2-3 kpc from the plane. The large majority of the stars with V = 13.5-15.5 are Population II field blue horizontal branch stars. Publication: The Astrophysical Journal Supplement Series Pub Date: December 1983 DOI: 10.1086/190910 Bibcode: 1983ApJS...53..791P Keywords: A Stars; Astronomical Spectroscopy; B Stars; Interstellar Matter; Radial Velocity; Blue Stars; Horizontal Branch Stars; Metallicity; Milky Way Galaxy; Southern Sky; Astrophysics full text sources ADS | data products SIMBAD (285) CDS (2) Related Materials (1) Catalog: 1995yCat..20530791P

A SURVEY FOR SMALL-AMPLITUDE VARIABILITY AMONG POPULATION II STARS.

ABSTRACT A search for small-amplitude variability among members of old stellar populations reveals no variability 0'!'01 among 15 metal-deficient or high-velocity F- and G-type subdwarfs, 13 old-disk main-sequence F stars, four cluster blue stragglers, and seven blue horizontal-branch field stars. Variability was detected in two A-type stars: A metal-deficient main-sequence A star, BDS 1269A = HD 15165 is shown to have 8 Scuti-like variability, and the variability of the high-velocity field blue straggler HD 100363 reported by Eggen (1971) is confirmed. The high-velocity sdAp star GD -210, a suspected variable, showed no amplitude 0'!'017 for three hours. Teff and log g of ten high-velocity, short-period variables are similar to those of the 8 Scuti stars with Teff 7800 K, although the low-mass variables in the sample probably do not share the same evolutionary history as the higher-mass field blue stragglers. Key words: stellar pulsation - subdwarfs - dwarf cepheids - 8 Scuti stars