20 research outputs found
Properties of interstellar dust in the region of the CEP OB4 association
Regional variations in the interstellar extinction law was examined. The region of Cyg was noted among the areas with abnormal interstellar extinction law (IEL). An attempt was made to study interstellar extinction in the region of the Cep OB4 association. The Vilnius seven color photometric system was used for this purpose. The effective wavelengths and their reciprocals for the Vilnius photometric system are presented. This system was applied for the study of variations in the IEL in four regions of the sky: Cyg, Cep, Per, and Mon (Sudzius, 1974). The results of multicolor photometry were combined with the data of Whiteoak (1966) and detailed interstellar extinction curves were derived for Cyg and for Cep-Per-Mon (average) regions in the wavelengths interval of 3000 to 8000 A (Sudzius, 1974). The obtained IEL for the Cep OB4 association does not match any of the laws determined for other regions of the sky. However, it is very close to Cyg OB2 law in the ultraviolet. Results suggest that Cep OB4 IEL have no very broadband structure (VBS) in the interval of wavenumbers 1.84 to 2.15/micron
Candidate Pre-Mainsequence F Stars with Circumstellar Dust Identified Using Combined 2MASS and uvby Data
We propose a method that uses near-infrared plus uvby photometry to identify
potentially extensive circumstellar dusty environment about F and A stars. The
method has been applied to a sample of ~900 metal rich reddened F stars with
2MASS and uvby data, suggesting the presence of circumstellar dust emitting in
the near infrared for ~70 stars. The log T_e - M_V diagram suggests that most,
if not all, of them are likely pre-mainsequence (PMS). They seem to be
consistent with being a continuation of the class of Herbig Ae/Be PMS stars
into the spectral type F. Their number drops sharply downward of log T_e ~ 3.84
(spectral types later than ~F5), which may provide new clues to the PMS
evolution of stars with 1 to 2 solar mass. We present a list of 21 most
conspicuous candidate stars with circumstellar dust. About half of them are
associated with the extended star-forming region around rho Oph. The brightest
of these 21 stars, with V < 7.5, turn out to be IRAS sources, suggesting the
presence of heated dust emitting in the far infrared. Also in this list, HD
81270 is reported as a very unusual star moving away from the Galactic plane at
a projected speed of 70 km/sec.Comment: 8 pages, 4 figures, 1 table. To appear in ApJ, part 2, v. 570, 2002
May
The Homogeneity of Interstellar Oxygen in the Galactic Disk
We present an analysis of high resolution HST Space Telescope Imaging
Spectrograph (STIS) observations of O I 1356 and H I Lyman-alpha absorption in
36 sight lines that probe a variety of Galactic disk environments and include
paths that range over nearly 4 orders of magnitude in f(H_2), over 2 orders of
magnitude in mean sight line density, and that extend up to 6.5 kpc in length.
Consequently, we have undertaken the study of gas-phase O/H abundance ratio
homogeneity using the current sample and previously published Goddard
High-Resolution Spectrograph (GHRS) results. Two distinct trends are identified
in the 56 sight line sample: an apparent decrease in gas-phase oxygen abundance
with increasing mean sight line density and a gap between the mean O/H ratio
for sight lines shorter and longer than about 800 pc. The first effect is a
smooth transition between two depletion levels associated with large mean
density intervals; it is centered near a density of 1.5 cm^-3 and is similar to
trends evident in gas-phase abundances of other elements. Paths less dense than
the central value exhibit a mean O/H ratio of log_10 (O/H) = -3.41+/-0.01 (or
390+/-10 ppm), which is consistent with averages determined for several long,
low-density paths observed by STIS (Andre et al. 2003) and short low-density
paths observed by FUSE (Moos et al. 2002). Sight lines of higher mean density
exhibit an average O/H value of log_10 (O/H) = -3.55+/-0.02 (284+/-12 ppm). The
datapoints for low-density paths are scattered more widely than those for
denser sight lines, due to O/H ratios for paths shorter than 800 pc that are
generally about 0.10 dex lower than the values for longer ones.Comment: 33 pages, including 8 figures and 4 tables; accepted for publication
in ApJ, tentatively in Oct 200
The effective temperature scale of FGK stars. II. Teff : color : [Fe/H] calibrations
We present up-to-date metallicity-dependent temperature vs. color
calibrations for main sequence and giant stars based on temperatures derived
with the infrared flux method (IRFM). Seventeen colors in the following
photometric systems: UBV, uvby, Vilnius, Geneva, RI(Cousins), DDO,
Hipparcos-Tycho, and 2MASS, have been calibrated. The spectral types covered
range from F0 to K5 (7000 K<Teff<4000 K) with some relations extending below
4000 K or up to 8000 K. Most of the calibrations are valid in the metallicity
range -3.5<[Fe/H]<0.4, although some of them extend to as low as [Fe/H]=-4.0.
All fits to the data have been performed with more than 100 stars; standard
deviations range from 30 K to 120 K. Fits were carefully performed and
corrected to eliminate the small systematic errors introduced by the
calibration formulae. Tables of colors as a function of Teff and [Fe/H] are
provided. (Abridged)Comment: To appear in ApJ. For online tables and figures, see
http://webspace.utexas.edu/ir68/tef