740 research outputs found
Supersoft X-Ray Sources in M31
The nearby Andromeda galaxy (M31) has been observed with the ROSAT PSPC in a
mosaic of 6 pointings with 25 ksec each. In the paper describing the results on
the total sample of detected sources, Supper et al. (1996) also report the
positions for 15 supersoft X-ray sources and the blackbody fit results for the
brightest of these sources. We report here in more detail on the X-ray spectral
characteristics of all these 15 supersoft X-ray sources. Optical multi-colour
photometric data obtained in 1990 at the Michigan- Dartmouth-MIT Observatory at
Kitt Peak were used originally in the selection process of the supersoft
sources, and are the basis for the finding charts given here for most selected
X-ray sources.Comment: 8 pages postscript incl. figures, Proc. of Workshop on Supersoft
X-Ray Sources, to appear in Lecture Notes in Physics vol. 472 (1996
The Stellar Population of the M31 Spiral Arm Around OB Association A24
A study of the stellar population of the M31 spiral arm around OB association
A24 was carried out based on the photometric data obtained from deep V and JHK
imaging. The luminosity function was obtained for -7 <~ Mbol <~ -3.5 by
applying the extinction correction corresponding to Av=1 and the bolometric
correction BC(K) as an empirical function of (J-K)o. In comparing the observed
color-luminosity diagrams with semitheoretical isochrones modified for the
dust-shell effects, we found the young population of t <~ 30 Myr with
supergiants of Mbol <~ -5, the bulk of the intermediate-age population of t ~
0.2 - 2.5 Gyr with bright asymptotic giant branch (AGB) stars of -5 <~ Mbol <~
-4, and old populations of t ~> 3 Gyr with AGB and red giant branch (RGB) stars
of Mbol ~> -4. The average star formation rate was estimated to be ~1.8x10^4
M_o/Myr and ~0.7x10^4 M_o/Myr per deprojected disk area of 1 kpc^2 from the
number density of B0 V stars around Mv=-4.0 (age ~10 Myr) and the number
density of bright AGB stars around Mbol = -4.3 (age ~1 Gyr), respectively. A
study of the local variation in the V and the J and H luminosity functions
revealed a kind of anticorrelation between the population of the young
component and that of the intermediate-age component when subdomains of ~100 pc
scales were concerned. This finding suggests that the disk domain around the
A24 area experienced a series of star formation episodes alternatively among
different subdomains with a timescale of a few spiral passage periods. Brief
discussions are given about the interstellar extinction and about the lifetimes
of bright AGB stars and the highly red objects (HROs) in the same area.Comment: 27 pages, 11 figures, accepted: ApJ, July 1, 199
Identification and Analysis of Young Star Cluster Candidates in M31
We present a method for finding clusters of young stars in M31 using
broadband WFPC2 data from the HST data archive. Applying our identification
method to 13 WFPC2 fields, covering an area of ~60 arcmin^2, has revealed 79
new candidate young star clusters in these portions of the M31 disk. Most of
these clusters are small (~<5 pc) young (~10-200 Myr) star groups located
within large OB associations. We have estimated the reddening values and the
ages of each candidate individually by fitting isochrones to the stellar
photometry. We provide a catalog of the candidates including rough
approximations of their reddenings and ages. We also look for patterns of
cluster formation with galactocentric distance, but our rough estimates are not
precise enough to reveal any clear patterns.Comment: 32 pages, 9 figures, 5 tables, accepted to Ap
Chandra Studies of the X-ray Point Source Luminosity Functions of M31
Three different M31 disk fields, spanning a range of stellar populations,
were observed by Chandra. We report the X-ray point source luminosity function
(LF) of each region, and the LF of M31's globular clusters, and compare these
with each other and with the LF of the galaxy's bulge. To interpret the results
we also consider tracers of the stellar population, such as OB associations and
supernova remnants. We find differences in the LFs among the fields, but cannot
definitively relate them to the stellar content of the fields. We find that
stellar population information, average and maximum source luminosities, X-ray
source densities, and slopes of the LF are useful in combination.Comment: 7 pages, 3 figures, accepted for publication in ApJ.
Higher-resolution figures available on reques
WFPC2 Observations of Massive and Compact Young Star Clusters in M31
We present color magnitude diagrams of four blue massive and compact star
clusters in M31: G38, G44, G94, and G293. The diagrams of the four clusters
reveal a well-populated upper main sequence and various numbers of supergiants.
The U-B and B-V colors of the upper main sequence stars are used to determine
reddening estimates of the different lines of sight in the M31 disk. Reddening
values range from E(B-V) = 0.20 +/- 0.10 to 0.31 +/- 0.11. We statistically
remove field stars on the basis of completeness, magnitude and color. Isochrone
fits to the field-subtracted, reddening-corrected diagrams yield age estimates
ranging from 63 +/- 15 Myr to 160 +/- 60 Myr. Implications for the recent
evolution of the disk near NGC 206 are discussed.Comment: 17 pages, Latex, ApJ, in Pres
Sealed operation, and circulation and purification of gas in the HARPO TPC
HARPO is a time projection chamber (TPC) demonstrator of a gamma-ray
telescope and polarimeter in the MeV-GeV range, for a future space mission. We
present the evolution of the TPC performance over a five month sealed-mode
operation, by the analysis of cosmic-ray data, followed by the fast and
complete recovery of the initial gas properties using a lightweight gas
circulation and purification system.Comment: Proceedings_MPGD2015, EPJ Web of Conference
X-ray Point Sources in The Central Region of M31 as seen by Chandra
We report on \chandra observations of the central region of M31. By combining
eight \chandra ACIS-I observations taken between 1999 and 2001, we have
identified 204 X-ray sources within the central region of
M31, with a detection limit of \lum. Of these 204 sources,
22 are identified with globular clusters, 2 with supernova remnants, 9 with
planetary nebula, and 9 as supersoft sources. By comparing individual images,
about 50% of the sources are variable on time scales of months. We also found
13 transients, with light curves showing a variety of shapes. We also extracted
the energy spectra of the 20 brightest sources; they can be well fit by a
single power-law with a mean photon index of 1.8. The spectral shapes of 12
sources are shown to be variable, suggesting that they went through state
changes. The luminosity function of all the point sources is consistent with
previous observations (a broken power-law with a luminosity break at
\lum). However, when the X-ray sources in different regions
are considered separately, different luminosity functions are obtained. This
indicates that the star-formation history might be different in different
regions.Comment: 42 pages, 9 figures, ApJ, accepted, Higher-resolution figures
available on reques
- …