1,825 research outputs found
A statistical analysis of the late-type stellar content in the Andromeda halo
We present a statistical characterization of the carbon-star to M-giant (C/M)
ratio in the halo of M31. Based on application of pseudo-filter band passes to
our Keck/DEIMOS spectra we measure the 81-77-color index of 1288 stars in the
giant stellar stream and in halo fields out to large distances. From this
well-established narrow-band system, supplemented by V-I colors, we find only a
low number (five in total) of C-star candidates. The resulting low C/M ratio of
10% is consistent with the values in the M31 disk and inner halo from the
literature. Although our analysis is challenged by small number statistics and
our sample selection, there is an indication that the oxygen-rich M-giants
occur in similar number throughout the entire halo. We also find no difference
in the C-star population of the halo fields compared to the giant stream. The
very low C/M ratio is at odds with the observed low metallicities and the
presence of intermediate-age stars at large radii. Our observed absence of a
substantial carbon star population in the these regions indicates that the
(outer) M31 halo cannot be dominated by the debris of disk-like or SMC-type
galaxies, but rather resemble the dwarf elliptical NGC 147.Comment: 11 pages, 10 figures, accepted for publication in the A
A chemical confirmation of the faint Bootes II dwarf Spheroidal Galaxy
We present a chemical abundance study of the brightest confirmed member star
of the ultrafaint dwarf galaxy Bootes II from Keck/HIRES high-resolution
spectroscopy at moderate signal-to-noise ratios. At [Fe/H] = -2.93 +/- 0.03
(stat.) +/- 0.17 (sys.) this star chemically resembles metal-poor halo field
stars and the signatures of other faint dwarf spheroidal galaxies at the same
metallicities in that it shows enhanced [alpha/Fe] ratios, Solar Fe-peak
element abundances, and low upper limits on the neutron-capture element Ba.
Moreover, this star shows no chemical peculiarities in any of the eight
elements we were able to measure. This implies that the chemical outliers found
in other systems remain outliers pertaining to the unusual enrichment histories
of the respective environments, while Bootes II appears to have experienced an
enrichment history typical of its very low mass. We also re-calibrated previous
measurements of the galaxy's metallicity from the calcium triplet (CaT) and
find a much lower value than reported before. The resulting broad metallicity
spread, in excess of one dex, the very metal poor mean, and the chemical
abundance patterns of the present star imply that Bootes II is a low-mass, old,
metal poor dwarf galaxy and not an overdensity associated with the Sagittarius
Stream as has been previously suggested based on its sky position and
kinematics. The low, mean CaT metallicity of -2.7 dex falls right on the
luminosity-metallicity relation delineated over four orders of magnitude from
the more luminous to the faintest galaxies. Thus Bootes II's chemical
enrichment appears representative of the galaxy's original mass, while tidal
stripping and other mass loss mechanisms were probably not significant as for
other low-mass satellites.Comment: 10 pages, 5 figures, accepted for publication in the Astrophysical
Journa
Rates for Color Shifted Microlensing Events
If the objects responsible for gravitational microlensing (ML) of
Galactic-bulge stars are faint dwarfs, then blended light from the lens will
distort the shape of the ML light curve and shift the color of the observed
star during the event. The resolution in current surveys is not accurate enough
to observe this effect, but it should be detected with frequent and precise
followup observations. We calculate the expected rates for ML events where the
shape distortions will be observable by such followup observations, assuming
that the lenses are ordinary main-sequence stars in a bar and in the disk. We
study the dependence of the rates for color-shifted (CS) events on the
frequency of followup observations and on the precision of the photometry for a
variety of waveband pairings. We find that for hourly observations in and
with typical photometric errors of 0.01 mag, 28\% of the events where a
main-sequence bulge star is lensed, and 7\% of the events where the source is a
bulge giant, will give rise to a measurable CS at the 95\% confidence level.
For observations in and , the fractions become 18\% and 5\%,
respectively, but may be increased to 40\% and 13\% by improved photometric
accuracy and increased sampling frequency. We outline how the mass, distance,
and transverse speed of the lens can be obtained, giving examples of typical
errors. We discuss how CS events can be distinguished from events where the
source is blended with a binary companion.Comment: 36 pages, uuencoded postscript fil
New High Proper Motion Stars from the Digitized Sky Survey. II. Northern Stars with 0.5<mu<2.0 arcsec/yr at High Galactic Latitudes
In a continuation of our systematic search for high proper motion stars in
the Digitized Sky Survey, we have completed the analysis of northern sky fields
at galactic latitudes above 25 degrees. With the help of our SUPERBLINK
software, a powerful automated blink comparator developed by us, we have
identified 1146 stars in the magnitude range 8<r<20 with proper motions
0.500<mu<2.000 arcsec/yr. These include 1080 stars previously listed in
Luyten's proper motion catalogs (LHS, NLTT), 9 stars not previously listed in
the Luyten catalogs but reported elsewhere in the literature (including 1
previously reported by our team), and 57 new objects reported here for the
first time. This paper includes a list of positions, proper motions,
magnitudes, and finder charts for all the new high proper motion stars.
Combined with our previous study of low galactic latitude fields (see Paper I),
our survey now covers over 98% of the northern sky. We conclude that the Luyten
catalogs were 90% complete in the northern sky for stars with 0.5<mu<2.0
arcsec/yr down to magnitude r=19. We discuss the incompleteness of the old
Luyten proper motion survey, and estimate completeness limits for our new
survey.Comment: To appear in The Astronomical Journa
K Giants in Baade's Window. II. The Abundance Distribution
This is the second in a series of papers in which we analyze spectra of over
400 K and M giants in Baade's Window, including most of the stars with proper
motions measured by Spaenhauer et al. [AJ, 103, 297 (1992)]. In our first
paper, we measured line--strength indices of Fe, Mg, CN and H and
calibrated them on the system of Faber et al. [ApJS, 57, 711 (1985)]. Here, we
use the index to derive an abundance distribution in
[Fe/H] for 322 stars with effective temperatures between 3900 K and 5160 K.
Our derived values of [Fe/H] agree well with those measured from
high--resolution echelle spectra (e.g., McWilliam \& Rich [ApJS, 91, 749
(1994)]) for the small number of stars in common. We find a mean abundance
for our sample of Baade's Window
K giants. More than half the sample lie in the range \feh\ .
We estimate line--of--sight distances for individual stars in our sample and
confirm that, in Baade's Window, most K giants with are foreground
disk stars, but the great majority (more than 80\%) with belong to the
bulge.
We also compare the metallicities derived from the CN and Mg indices to
those from iron. Most of the metal--rich stars in our sample appear to be
CN--weak, in contrast to the situation in metal--rich globular clusters and
elliptical galaxies. The metal--poor half of our sample ([Fe/H] ) shows
evidence for a mild Mg overenhancement ([Mg/Fe] ); but this is not
seen in the more metal--rich stars ([Fe/H] 0). The K giants in Baade's
Window therefore share some, but not all, of the characteristics of stars in
elliptical galaxies as inferred from their integrated light.Comment: Accepted for publication in the Astronomical Journal, tentatively
scheduled for July, 1996. LaTex source which generates 40 pages of text (no
figures or tables). Complete (text + 15 figs + 5 tables) preprint in gzip/tar
format is also available at
ftp://bessel.mps.ohio-state.edu/pub/terndrup/kg2.tar.gz (227 kbyte
- …