796 research outputs found
DIRECT Distances to Nearby Galaxies Using Detached Eclipsing Binaries and Cepheids. V. Variables in the Field M31F
We undertook a long term project, DIRECT, to obtain the direct distances to
two important galaxies in the cosmological distance ladder -- M31 and M33 --
using detached eclipsing binaries (DEBs) and Cepheids. While rare and difficult
to detect, DEBs provide us with the potential to determine these distances with
an accuracy better than 5%. The extensive photometry obtained in order to
detect DEBs provides us with good light curves for the Cepheid variables. These
are essential to the parallel project to derive direct Baade-Wesselink
distances to Cepheids in M31 and M33. For both Cepheids and eclipsing binaries,
the distance estimates will be free of any intermediate steps.
As a first step in the DIRECT project, between September 1996 and October
1997 we obtained 95 full/partial nights on the F. L. Whipple Observatory 1.2 m
telescope and 36 full nights on the Michigan-Dartmouth-MIT 1.3 m telescope to
search for DEBs and new Cepheids in the M31 and M33 galaxies. In this paper,
fifth in the series, we present the catalog of variable stars found in the
field M31F [(\alpha,\delta)= (10.\arcdeg10, 40.\arcdeg72), J2000.0]. We have
found 64 variable stars: 4 eclipsing binaries, 52 Cepheids and 8 other
periodic, possible long period or non-periodic variables. The catalog of
variables, as well as their photometry and finding charts, is available via
anonymous ftp and the World Wide Web. The complete set of the CCD frames is
available upon request.Comment: submitted to the Astronomical Journal, 31 pages, 18 figures; paper
and data available at ftp://cfa-ftp.harvard.edu/pub/kstanek/DIRECT/ and
through WWW at http://cfa-www.harvard.edu/~kstanek/DIRECT
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
DIRECT Distances to Nearby Galaxies Using Detached Eclipsing Binaries and Cepheids. III. Variables in the Field M31C
We undertook a long term project, DIRECT, to obtain the direct distances to
two important galaxies in the cosmological distance ladder -- M31 and M33 --
using detached eclipsing binaries (DEBs) and Cepheids. While rare and difficult
to detect, DEBs provide us with the potential to determine these distances with
an accuracy better than 5%. The extensive photometry obtained in order to
detect DEBs provides us with good light curves for the Cepheid variables. These
are essential to the parallel project to derive direct Baade-Wesselink
distances to Cepheids in M31 and M33. For both Cepheids and eclipsing binaries,
the distance estimates will be free of any intermediate steps.
As a first step in the DIRECT project, between September 1996 and October
1997 we obtained 95 full/partial nights on the F. L. Whipple Observatory 1.2 m
telescope and 36 full nights on the Michigan-Dartmouth-MIT 1.3 m telescope to
search for DEBs and new Cepheids in the M31 and M33 galaxies. In this paper,
third in the series, we present the catalog of variable stars, most of them
newly detected, found in the field M31C [(alpha,delta)=(11.10, 41.42) deg,
J2000.0}]. We have found 115 variable stars: 12 eclipsing binaries, 35 Cepheids
and 68 other periodic, possible long period or non-periodic variables. The
catalog of variables, as well as their photometry and finding charts, is
available via anonymous ftp and the World Wide Web. The complete set of the CCD
frames is available upon request.Comment: submitted to the Astronomical Journal, 39 pages, 27 figures; paper
and data available at ftp://cfa-ftp.harvard.edu/pub/kstanek/DIRECT/ and
through WWW at http://cfa-www.harvard.edu/~kstanek/DIRECT
On Estimating the QSO Transmission Power Spectrum
We present new methods to minimize the systematic and random errors for
measuring the transmission power spectrum from the Lyman-alpha forest. Sources
of systematic errors explored include metal line contamination and
continuum-fitting. We advocate the technique of trend-removal in place of
traditional continuum-fitting -- here, a spectrum is normalized by its
(smoothly varying) mean rather than its continuum -- this method is easily
automated and removes biases introduced by continuum-fitting. Trend- removal
can be easily applied to spectra where continuum-fitting is difficult, such as
when the resolution or signal-to-noise is low, or for spectra at high
redshifts. Furthermore, a measurement of the continuum power spectrum using
trend-removal, from either low redshift quasar spectra or the red-side of
Lyman-alpha, allows in principle the removal of spurious power introduced by
the continuum and thereby expanding scales probed to larger ones. We also
derive expressions for the shot-noise bias and variance of the power spectrum
estimate, taking into account the non-Poissonian nature of the shot-noise and
the non-Gaussianity of the cosmic fluctuations. An appropriate minimum variance
weighting of the data is given. Finally, we give practical suggestions on
observing strategy: the desired resolution and S/N for different purposes, and
how to distribute one's finite observing time among quasar targets. Also
discussed is the quasar spectroscopic study of the Sloan Digital Sky Survey,
which has the potential to measure the power spectrum at z ~ 2-4 accurate to
better than 1 % per mode -- the techniques presented here will be useful for
tackling the anticipated issues of shot-noise and continuum contamination.Comment: 35 pages, 14 figures, submitted 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
The DIRECT project: Catalogs of stellar objects in nearby galaxies. II. Eastern arm and NGC 206 in M31
DIRECT is a project to directly obtain the distances to two important
galaxies in the cosmological distance ladder, M31 and M33, using detached
eclipsing binaries and Cepheids. As part of our search for these variables, we
have obtained photometry and positions for thousands of stellar objects within
the monitored fields, covering an area of 557.8 arcmin^2. In this research note
we present the equatorial coordinates and BVI photometry for 26712 stars in the
M31 galaxy, along the eastern arm and in the vicinity of the star forming
region NGC206.Comment: 2 LaTeX pages, 2 Postscript figures, submitted to Astronomy &
Astrophysic
A Survey of Local Group Galaxies Currently Forming Stars. I. UBVRI Photometry of Stars in M31 and M33
We present UBVRI photometry obtained from Mosaic images of M31 and M33 using
the KPNO 4-m telescope. The survey covers 2.2 sq degrees of M31, and 0.8 sq
degrees of M33, chosen so as to include all of the regions currently active in
forming massive stars. The catalog contains 371,781 and 146,622 stars in M31
and M33, respectively, where every star has a counterpart (at least) in B, V,
and R. We compare our photometry to previous studies. We provide cross
references to the stars confirmed as members by spectroscopy, and compare the
location of these to the complete set in color-magnitude diagrams. While
follow-up spectroscopy is needed for many projects, we demonstrate the success
of our photometry in being able to distinguish M31/M33 members from foreground
Galactic stars. We also present the results of newly obtained spectroscopy,
which identifies 34 newly confirmed members, including B-A supergiants, the
earliest O star known in M31, and two new Luminous Blue Variable candidates
whose spectra are similar to that of P Cygni.Comment: Accepted by the Astronomical Journal. A version with higher
resolution figures can be found at:
http://www.lowell.edu/users/massey/M3133.pdf.g
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
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
Four Brown Dwarfs in the Taurus Star-Forming Region
We have identified four brown dwarfs in the Taurus star-forming region. They
were first selected from and CCD photometry of 2.29 square degrees
obtained at the Canada-France-Hawaii Telescope. Subsequently, they were
recovered in the 2MASS second incremental data release point source catalog.
Low-resolution optical spectra obtained at the William Herschel telescope allow
us to derive spectral types in the range M7--M9. One of the brown dwarfs has
very strong H emission (EW=-340 \AA). It also displays Br
emission in an infrared spectrum obtained with IRCS on the Subaru telescope,
suggesting that it is accreting matter from a disk. The \ion{K}{1} resonance
doublet and the \ion{Na}{1} subordinate doublet at 818.3 and 819.5 nm in these
Taurus objects are weaker than in field dwarfs of similar spectral type,
consistent with low surface gravities as expected for young brown dwarfs. Two
of the objects are cooler and fainter than GG Tau Bb, the lowest mass known
member of the Taurus association. We estimate masses of only 0.03 M for
them. The spatial distribution of brown dwarfs in Taurus hints to a possible
anticorrelation between the density of stars and the density of brown dwarfs.Comment: ApJ Letters (in press
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