88 research outputs found
Physical parameters of the Cen X-3 system
Photographic spectra of Cen X-3 show that the primary star has a spectral type near 06.5 with weak, variable emission at wavelength 4640 and 4686. No orbital motion of the emission or absorption lines is detected; for the latter the upper limit is approximately + or - 50 km/s. Analysis of the available data indicates that the primary is a factor of 2-3 less massive than expected from normal evolutionary models while the X-ray source has a solar mass near 1.5
A Deep Multicolor Survey V: The M Dwarf Luminosity Function
We present a study of M dwarfs discovered in a large area, multicolor survey.
We employ a combination of morphological and color criteria to select M dwarfs
to a limiting magnitude in V of 22, the deepest such ground-based survey for M
dwarfs to date. We solve for the vertical disk stellar density law and use the
resulting parameters to derive the M dwarf luminosity and mass functions from
this sample. We find the stellar luminosity function peaks at M_V = 12 and
declines thereafter. Our derived mass function for stars with M < 0.6 M_sun is
inconsistent with a Salpeter function at the 3 sigma level; instead, we find
the mass function is relatively flat for 0.6 M_sun > M > 0.1 M_sun.Comment: Accepted for publication in AJ. 19 pages including 4 embedded
postscript figures (AASTEX
An Infrared Search for Star-Forming Galaxies at z > 2
We report the cumulative results of an on-going near-infrared search for
redshifted H-alpha emission from normal galaxies at z>2. An infrared search
reduces the bias due to reddening. Using narrow-band imaging with the Near
Infrared Camera on the Keck I 10-m telescope, a survey area of almost 12 square
arcminutes has been covered. Target regions were selected by matching the
redshifts of QSO emission and metal-line absorptions to our available filters.
The survey depth is 7E-17 ergs/cm^2/s (3sigma) in H-alpha and K-prime ~22.
Eleven H-alpha-emitters, plus two Seyfert I objects, have been discovered. The
high density of galaxy detections, corresponding to a co-moving volume density
of 0.0135/Mpc^3, makes it unlikely that all of the H-alpha flux in these
objects is the result of active nuclei. There is a strong suggestion of
clustering in the environments of metal-line absorbers. Each candidate galaxy
lies typically within a projected distance of 250kpc of the QSO line of sight
and is resolved but compact. The average Star Formation Rate inferred for the
galaxies from the H-alpha flux is 50 Msun/yr, significantly higher than current
day star-forming galaxies, but consistent with other estimates for galaxies at
high redshift.Comment: 39 pages including 17 figures, accepted for publication in Ap
XMM-Newton Observations of High Redshift Quasars
We report on our XMM observations of the high redshift quasars BR 2237--0607
(z=4.558) and BR 0351--1034 (z=4.351), together with 14 other z>4 objects found
in the XMM public archive. Contrary to former reports, we do not find high
redshift radio-loud quasars to be more absorbed than their radio-quiet
counterparts. We find that the optical to X-ray spectral index alpha-ox is
correlated with the luminosity density at 2500 A, but does not show a
correlation with redshift. The mean 2-10 keV power-law slope of the 9 high
redshift radio-quiet quasars in our sample for which a spectral analysis can be
performed is alpha-x1.23+-0.48, similar to alpha-x=1.19 found from the ASCA
observations of low redshift Narrow-Line Seyfert 1 galaxies (NLS1s), and
significantly different from alpha-x=0.78 found for low redshift Broad-Line
Seyfert galaxies. While the optical/UV spectra of low to high redshift quasars
look remarkably similar, we find a first indication of a difference in their
X-ray spectrum. The steep X-ray spectral index suggests high Eddington ratios
L/L_Edd. These observations give credence to the hypothesis of Mathur (2000)
that NLS1s are low luminosity cousins of high redshift quasars, both likely to
be in their early evolutionary stage.Comment: 25 pages, AJ, in press (Jan 2006
Probing populations of red giants in the galactic disk with CoRoT
The detection with CoRoT of solar-like oscillations in nearly 800 red giants
in the first 150-days long observational run paves the way for detailed studies
of populations of galactic-disk red giants. We investigate which information on
the observed population can be recovered by the distribution of the observed
seismic constraints: the frequency of maximum oscillation power (nu_max) and
the large frequency separation (Deltanu). We propose to use the observed
distribution of nu_max and of Deltanu as a tool for investigating the
properties of galactic red-giant stars through comparison with simulated
distributions based on synthetic stellar populations. We can clearly identify
the bulk of the red giants observed by CoRoT as red-clump stars, i.e.
post-flash core-He-burning stars. The distribution of nu_max and of Deltanu
gives us access to the distribution of the stellar radius and mass, and thus
represent a most promising probe of the age and star formation rate of the
disk, and of the mass-loss rate during the red-giant branch.
CoRoT observations are supplying seismic constraints for the most populated
class of He-burning stars in the galactic disk. This opens a new access gate to
probing the properties of red-giant stars that, coupled with classical
observations, promises to extend our knowledge of these advanced phases of
stellar evolution and to add relevant constraints to models of composite
stellar populations in the Galaxy.Comment: 4 pages, 6 figures, accepted in A&A Letter
Quasar Candidates in the Hubble Deep Field
We focus on the search for unresolved faint quasars and AGN in the crude
combine images using a multicolor imaging analysis that has proven very
successful in recent years. Quasar selection was carried out both in multicolor
space and in "profile space," defined as the multi-parameter space formed by
the radial profiles of the objects in the different images. By combining the
dither frames available for each filter, we were able to obtain well-sampled
radial profiles of the objects and measure their deviation from that of a
stellar source. We also generated synthetic quasar spectra in the range 1.0 < z
< 5.5 and computed expected quasar colors. We determined that the data are 90%
complete for point sources at 26.2, 28.0, 27.8, 26.8 in the F300W, F450W, F606W
and F814W filters, respectively. We find 41 compact objects in the HDF: 8
pointlike objects with colors consistent with quasars or stars, 18 stars, and
15 slightly resolved objects, 12 of which have colors consistent with quasars
or stars. We estimate the upper limit of unresolved and slightly resolved
quasars/AGNs with V < 27.0 and z < 3.5 to be 20 objects (16,200 per deg^2). We
find good agreement among authors on the number of stars and the lack of quasar
candidates with z > 3.5. We find more quasar candidates than previous work
because of our more extensive modeling and use of all of the available color
information. (abridged)Comment: We have clarified our discussion and conclusions, added some
references and removed the appendix, which is now available from the first
author. 37 pages including 10 embedded postscript figures and 6 tables. To
appear in the Feb. 99 issue of A
- …