26 research outputs found
The Hobby-Eberly Telescope Chemical Abundances Of Stars In The Halo (CASH) Project. I. The Lithium-, s-, And r-Enhanced Metal-Poor Giant HKII 17435-00532
We present the first detailed abundance analysis of the metal-poor giant HKII 17435-00532. This star was observed as part of the University of Texas long-term project Chemical Abundances of Stars in the Halo ( CASH). A spectrum was obtained with the High Resolution Spectrograph (HRS) on the Hobby-Eberly Telescope with a resolving power of R similar to 15,000. Our analysis reveals that this star may be located on the red giant branch, red horizontal branch, or early asymptotic giant branch. We find that this metal-poor (Fe/H = -2.2) star has an unusually high lithium abundance [log epsilon(Li) +2.1], mild carbon (C/Fe = +0.7) and sodium (]Na/Fe] = +0.6) enhancement, as well as enhancement of both s-process ([Ba/Fe] = +0.8) and r-process ([Eu/Fe] = +0.5) material. The high Li abundance can be explained by self-enrichment through extra mixing that connects the convective envelope with the outer regions of the H-burning shell. If so, HKII 17435-00532 is the most metal-poor star in which this short-lived phase of Li enrichment has been observed. The Na and n-capture enrichment can be explained by mass transfer from a companion that passed through the thermally pulsing AGB phase of evolution with only a small initial enrichment of r-process material present in the birth cloud. Despite the current nondetection of radial velocity variations (over similar to 180 days), it is possible that HKII 17435 - 00532 is in a long-period or highly inclined binary system, similar to other stars with similar n-capture enrichment patterns.NASA AAS Small Research Grant ProgramGALEX GI 05-GALEX05-27Italian MIUR-PRIN06 ProjectNSF AST 06-07708, AST04-06784, AST 07-0776, PHY 02-15783JINA AST 07-07447Astronom
Discovery of HE 1523-0901, a Strongly r-Process Enhanced Metal-Poor Star with Detected Uranium
We present age estimates for the newly discovered very r-process enhanced
metal-poor star HE 1523-0901 ([Fe/H]=-2.95) based on the radioactive decay of
Th and U. The bright (V=11.1) giant was found amongst a sample of bright
metal-poor stars selected from the Hamburg/ESO survey. From an abundance
analysis of a high-resolution (R=75,000) VLT/UVES spectrum we find HE 1523-0901
to be strongly overabundant in r-process elements ([r/Fe]=1.8). The abundances
of heavy neutron-capture elements (Z>56) measured in HE 1523-0901 match the
scaled solar r-process pattern extremely well. We detect the strongest optical
U line at 3859.57 A. For the first time, we are able to employ several
different chronometers, such as the U/Th, U/Ir, Th/Eu and Th/Os ratios to
measure the age of a star. The weighted average age of HE 1523-0901 is 13.2
Gyr. Several sources of uncertainties are assessed in detail.Comment: 7 pages, 2 figures. Accepted for publication in ApJ Letter
A Two Micron All-Sky Survey View of the Sagittarius Dwarf Galaxy: II. Swope Telescope Spectroscopy of M Giant Stars in the Dynamically Cold Sagittarius Tidal Stream
We present moderate resolution (~6 km/s) spectroscopy of 284 M giant
candidates selected from the Two Micron All Sky Survey photometry. Radial
velocities (RVs) are presented for stars mainly in the south, with a number
having positions consistent with association to the trailing tidal tail of the
Sagittarius (Sgr) dwarf galaxy. The latter show a clear RV trend with orbital
longitude, as expected from models of the orbit and destruction of Sgr. A
minimum 8 kpc width of the trailing stream about the Sgr orbital midplane is
implied by verified RV members. The coldness of this stream (dispersion ~10
km/s) provides upper limits on the combined contributions of stream heating by
a lumpy Galactic halo and the intrinsic dispersion of released stars, which is
a function of the Sgr core mass. The Sgr trailing arm is consistent with a
Galactic halo containing one dominant, LMC-like lump, however some lumpier
halos are not ruled out. An upper limit to the total M/L of the Sgr core is 21
in solar units. A second structure that roughly mimics expectations for
wrapped, leading Sgr arm debris crosses the trailing arm in the Southern
Hemisphere; however, this may also be an unrelated tidal feature. Among the <13
kpc M giants toward the South Galactic Pole are some with large RVs that
identify them as halo stars, perhaps part of the Sgr leading arm near the Sun.
The positions and RVs of Southern Hemisphere M giants are compared with those
of southern globular clusters potentially stripped from the Sgr system and
support for association of Pal 2 and Pal 12 with Sgr debris is found. Our
discussion includes description of a masked-filtered cross-correlation
methodology that achieves better than 1/20 of a resolution element RVs in
moderate resolution spectra.Comment: 41 pages, 6 figures, Astronomical Journal, in press (submitted Nov.
24, 2003; tentatively scheduled for July 2004 issue
Broadband UBVR_CI_C Photometry of Horizontal-Branch and Metal-poor Candidates from the HK and Hamburg/ESO Surveys. I.
We report broadband UBV and/or BVR_CI_C CCD photometry for a total of 1857 stars in the thick-disk and halo populations of the Galaxy. The majority of our targets were selected as candidate field horizontal-branch or other A-type stars (FHB/A, N = 576), or candidate low-metallicity stars (N = 1221), from the HK and Hamburg/ESO objective-prism surveys. Similar data for a small number of additional stars from other samples are also reported. These data are being used for several purposes. In the case of the FHB/A candidates they are used to accurately separate the lower gravity FHB stars from various higher gravity A-type stars, a subsample that includes the so-called blue metal poor stars, halo and thick-disk blue stragglers, main-sequence A-type dwarfs, and Am and Ap stars. These data are also being used to derive photometric distance estimates to high-velocity hydrogen clouds in the Galaxy and for improved measurements of the mass of the Galaxy. Photometric data for the metal-poor candidates are being used to refine estimates of stellar metallicity for objects with available medium-resolution spectroscopy, to obtain distance estimates for kinematic analyses, and to establish initial estimates of effective temperature for analysis of high-resolution spectroscopy of the stars for which this information now exists
Broadband UBVRI Photometry of Horizontal-Branch and Metal-Poor Candidates from the HK and Hamburg/ESO Surveys. I
We report broadband UBV and/or BVRI CCD photometry for a total of 1857 stars
in the thick-disk and halo populations of the Galaxy. The majority of our
targets were selected as candidate field horizontal-branch or other A-type
stars (FHB/A, N = 576), or candidate low-metallicity stars (N = 1221), from the
HK and Hamburg/ESO objective-prism surveys. Similar data for a small number of
additional stars from other samples are also reported.
These data are being used for several purposes. In the case of the FHB/A
candidates they are used to accurately separate the lower-gravity FHB stars
from various higher-gravity A-type stars, a subsample that includes the
so-called Blue Metal Poor stars, halo and thick-disk blue stragglers,
main-sequence A-type dwarfs, and Am and Ap stars. These data are also being
used to derive photometric distance estimates to high-velocity hydrogen clouds
in the Galaxy and for improved measurements of the mass of the Galaxy.
Photometric data for the metal-poor candidates are being used to refine
estimates of stellar metallicity for objects with available medium-resolution
spectroscopy, to obtain distance estimates for kinematic analyses, and to
establish initial estimates of effective temperature for analysis of
high-resolution spectroscopy of the stars for which this information now
exists.Comment: 22 pages, including 3 figures, 5 tables, and two ascii files of full
data, accepted for publication in the Astrophysical Journal (Supplements
Dynamically Tagged Groups of Very Metal-poor Halo Stars from the HK and Hamburg/ESO Surveys
© 2020. The American Astronomical Society. All rights reserved.We analyze the dynamical properties of âŒ1500 very metal-poor (VMP; [Fe/H] âČâ2.0) halo stars, based primarily on medium-resolution spectroscopic data from the HK and Hamburg/ESO surveys. These data, collected over the past thirty years, are supplemented by a number of calibration stars and other small samples, along with astrometric information from Gaia DR2. We apply a clustering algorithm to the 4-D energy-action space of the sample, and identify a set of 38 Dynamically Tagged Groups (DTGs), containing between 5 and 30 member stars. Many of these DTGs can be associated with previously known prominent substructures such as Gaia-Sausage/Enceladus (GSE), Sequoia, the Helmi Stream (HStr), and Thamnos. Others are associated with previously identified smaller dynamical groups of stars and streams. We identify 10 new DTGs as well, many of which have strongly retrograde orbits. We also investigate possible connections between our DTGs and âŒ300 individual r-process-enhanced (RPE) stars \textbf{from a recent literature compilation}. We find that several of these objects have similar dynamical properties to GSE (5), the HStr (4), Sequoia (1), and Rg5 (1), indicating that their progenitors might have been important sources of RPE stars in the Galaxy. Additionally, a number of our newly identified DTGs are shown to be associated with at least two RPE stars each (DTG-2: 3, DTG-7: 2; DTG-27: 2). Taken as a whole, these results are consistent with ultra-faint and/or dwarf spheroidal galaxies as birth environments in which r-process nucleosynthesis took place, and then were disrupted by the Milky Way.Peer reviewe