254 research outputs found
The distance to NGC 6397 by M-subdwarf main-sequence fitting
Recent years have seen a substantial improvement both in photometry of low
luminosity stars in globular clusters and in modelling the stellar atmospheres
of late-type dwarfs. We build on these observational and theoretical advances
in undertaking the first determination of the distance to a globular cluster by
main-sequence fitting using stars on the lower main sequence. The calibrating
stars are extreme M subdwarfs, as classified by Gizis (1997), with parallaxes
measured to a precision of better than 10%. Matching against King et al's
(1998) deep (V, (V-I)) photometry of NGC 6397, and adopting E_{B-V}=0.18 mag,
we derive a true distance modulus of 12.13 +- 0.15 mag for the cluster. This
compares with (m-M)_0=12.24 +- 0.1 derived through conventional main-sequence
fitting in the (V, (B-V)) plane. Allowing for intrinsic differences due to
chemical composition, we derive a relative distance modulus of delta
(m-M)_0=2.58 mag between NGC 6397 and the fiducial metal-poor cluster M92. We
extend this calibration to other metal-poor clusters, and examine the resulting
RR Lyrae (M_V, [Fe/H]) relation.Comment: 19 pages, AASTeX, to appear in the December 1998 A
Younger and brighter - New distances to globular clusters based on Hipparcos parallax measurements of local subdwarfs
We have used new parallax measurements, obtained by the Hipparcos satellite,
of fifteen nearby, metal-poor stars to re-define the subdwarf main-sequence.
All of these stars have parallaxes determined to an accuracy of at least 12 %.
Comparing these measurements against previous ground-based data for nine stars
reveals a systematic offset of 5 %, in the sense that the Hipparcos parallaxes
are smaller (i.e. the inferred distances are larger). The availability of the
Hipparcos observations expands the local subdwarf sample to the extent that we
can separate the stars by abundance into intermediate ([Fe/H] ~ -1.4) and
extreme ([Fe/H] ~ -2) subsets. Main-sequence fitting techniques are then used
to match stars of the appropriate abundance range to the colour-magnitude
diagrams of the seven globular clusters M5, NGC 6752, M13, M15, M92, M30 and
M68. We derive respective distance moduli of 14.45, 13.17, 14.48, 15.38, 14.93,
14.95 and 15.29 magnitudes, with formal uncertainties of +/- 0.1 magnitude. The
metal-poor systems M68, M15 and M30 have moderate foreground reddening, and
varying E(B-V) by +/- 0.02 magnitudes can change the derived distances by up to
+/- 7 %. With the exception of NGC 6752, however, our derived distances exceed
previous estimates, particularly in the case of the four [Fe/H] ~ -2.1
globulars, where our distance moduli are ~ 0.3 magnitudes higher than the
current standard values. We discuss briefly how these findings affect the RR
Lyrae distance scale, isochrone-based estimates of the age of globular clusters
and our picture of the early stages of star formation in the Galaxy. We note
that our results go some way towards reconciling the apparent contradiction
between the cluster ages and recent determinations of the Hubble constant.Comment: to appear in AJ; 47 pages, including 13 embedded, postscript figures
and 3 tables; uses AAS LaTeX style files (not included
Probing the LHS Catalog. I. New Nearby Stars and the Coolest Subdwarf
We present moderate resolution spectroscopy of 112 cool dwarf stars to
supplement the observations we have already presented in the Palomar/MSU
Nearby-Star Spectroscopic Survey. The sample consists of 72 suspected nearby
stars added to the The Preliminary Third Catalog of Nearby Stars since 1991 as
well as 40 faint red stars selected from the LHS catalog. LHS 1826 is more
metal-poor and cooler than the coolest previously known extreme subdwarf, LHS
1742a. LHS 2195 is a very late M dwarf of type M8 V, probably at a distance of
ten parsecs. LHS 1937 is an M7 V star at 20 parsecs. Three other previously
unobserved LHS stars have estimated distances that place them within 25
parsecs.Comment: 15 pages, 1 figure, AASTeX, to appear in the August 1997 PAS
Low-mass Binaries in the Hyades - A scarcity of brown dwarfs
We have obtained HST Planetary Camera observations of a total of fifty-three
low-mass (M < 0.3 M_solar) members of the Hyades cluster. Nine of these stars
are resolved as binaries, with separations between 0.1 and 3.1 arcseconds,
while a further three are probably equal-mass systems at smaller separations.
Allowing for observational selection effects, this corresponds to an observed
binary fraction of 11.3 plus or minus 4.6% for systems with separations in the
range 14 to 825 a.u., consistent with observations of solar neighbourhood
M-dwarfs. The mass-ratio distribution is only consistent marginally with the
secondary stars being drawn from a Psi(M) proportioal to M^{-1} mass function,
with three equal-mass systems amongst the six binaries with observed
separations in the regime where our observations extend below the
hydrogen-burning limit. Considering the entire sample, the absence of any brown
dwarf companions amongst our sample makes it unlikely that the mass function of
stellar/brown dwarf companions rises as steeply as M^{-1}. If the Hyades has an
age of 600 Myrs, our results are consistent only at the 2 sigma level with a
flat (M^0) mass function.Comment: 19 pages, 2 figures, AASTeX, to appear in the December 1997 A
FUSE, STIS, and Keck spectroscopic analysis of the UV-bright star vZ 1128 in M3 (NGC 5272)
We present a spectral analysis of the UV-bright star vZ 1128 in M3 based on
observations with the Far Ultraviolet Spectroscopic Explorer (FUSE), the Space
Telescope Imaging Spectrograph (STIS), and the Keck HIRES echelle spectrograph.
By fitting the H I, He I, and He II lines in the Keck spectrum with non-LTE
H-He models, we obtain Teff = 36,600 K, log g = 3.95, and log N(He)/N(H) =
-0.84. The star's FUSE and STIS spectra show photospheric absorption from C, N,
O, Al, Si, P, S, Fe, and Ni. No stellar features from elements beyond the iron
peak are observed. Both components of the N V 1240 doublet exhibit P~Cygni
profiles, indicating a weak stellar wind, but no other wind features are seen.
The star's photospheric abundances appear to have changed little since it left
the red giant branch (RGB). Its C, N, O, Al, Si, Fe, and Ni abundances are
consistent with published values for the red-giant stars in M3, and the
relative abundances of C, N, and O follow the trends seen on the cluster RGB.
In particular, its low C abundance suggests that the star left the asymptotic
giant branch before the onset of third dredge-up.Comment: 15 pages, 10 figures, to be published in MNRA
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