317 research outputs found
Thermal Conductivity of the Spin Peierls Compound CuGeO_3
The thermal conductivity of the Spin-Peierls (SP) compound CuGeO_3 was
measured in magnetic fields up to 16 T. Above the SP transition, the heat
transport due to spin excitations causes a peak at around 22 K, while below the
transition the spin excitations rapidly diminish and the heat transport is
dominated by phonons; however, the main scattering process of the phonons is
with spin excitations, which demonstrates itself in an unusual peak in the
thermal conductivity at about 5.5 K. This low-temperature peak is strongly
suppressed with magnetic fields in excess of 12.5 T.Comment: 6 pages, including 2 postscript figure
Chandra Study of the Cepheus B Star Forming Region: Stellar Populations and the Initial Mass Function
Cepheus B (Cep B) molecular cloud and a portion of the nearby Cep OB3b OB
association, one of the most active regions of star formation within 1 kpc, has
been observed with the ACIS detector on board the Chandra X-ray Observatory. We
detect 431 X-ray sources, of which 89% are confidently identified as clustered
pre-main sequence stars. Two main results are obtained. First, we provide the
best census to date for the stellar population of the region. We identify many
members of two rich stellar clusters: the lightly obscured Cep OB3b
association, and the deeply embedded cluster in Cep B whose existence was
previously traced only by a handful of radio sources and T Tauri stars. Second,
we find a discrepancy between the X-ray Luminosity Functions of the Cep OB3b
and the Orion Nebula Cluster. This may be due to different Initial Mass
Functions of two regions (excess of ~0.3 solar mass stars), or different age
distributions. Several other results are obtained. A diffuse X-ray component
seen in the field is attributed to the integrated emission of unresolved low
mass PMS stars. The X-ray emission from HD 217086 (O7n), the principle ionizing
source of the region, follows the standard model involving many small shocks in
an unmagnetized radiatively accelerated wind. The X-ray source #294 joins a
number of similar superflare PMS stars where long magnetic structures may
connect the protoplanetary disk to the stellar surface.Comment: 72 pages, 31 figures, 8 tables. Accepted for publication in Ap
Optical spectroscopy of X-ray sources in the Taurus molecular cloud: discovery of ten new pre-main sequence stars
We have analyzed optical spectra of 25 X-ray sources identified as potential
new members of the Taurus molecular cloud (TMC), in order to confirm their
membership in this SFR. Fifty-seven candidates were previously selected among
the X-ray sources in the XEST survey, having a 2MASS counterpart compatible
with a PMS star based on color-magnitude and color-color diagrams. We obtained
high-resolution optical spectra for 7 of these candidates with the SARG
spectrograph at the TNG telescope, which were used to search for Li absorption
and to measure the Ha line and the radial and rotational velocities; 18
low-resolution optical spectra obtained with DOLORES for other candidate
members were used for spectral classification, for Ha measurements, and to
assess membership together with IR color-color and color-magnitude diagrams and
additional information from the X-ray data. We found that 3 sources show Li
absorption, with equivalent widths of ~500 mA, broad spectral line profiles,
indicating v sin i ~20-40 km/s, radial velocities consistent with those for
known members, and Ha emission. Two of them are classified as new WTTSs, while
the EW (~ -9 Ang) of the Ha line and its broad asymmetric profile clearly
indicate that the third star (XEST-26-062) is a CTTS. Fourteen sources observed
with DOLORES are M-type stars. Fifteen sources show Ha emission; 6 of them have
spectra that indicate surface gravity lower than in MS stars, and their
de-reddened positions in IR color-magnitude diagrams are consistent with their
derived spectral type and with PMS models at the distance of the TMC. The
K-type star XEST-11-078 is confirmed as a new member from the strength of its
Ha emission line. Overall, we confirm membership to the TMC for 10 out of 25
X-ray sources observed in the optical. Three sources remain uncertain.Comment: 15 pages, 7 figures, accepted by Astronomy & Astrophysic
Radio-loud Active Galaxies in the Northern ROSAT All-Sky Survey III: New Spectroscopic Identifications from the RGB BL Lac Survey
We present new spectroscopic identifications for 169 objects in the
RASS-Green Bank (RGB) catalog of radio- and X-ray-emitting AGN. These data
significantly increase the fraction of bright RGB objects with classifications.
Specifically, we report and discuss the classification of 66 radio-loud
quasars, 53 BL Lacs, 33 Broad Line Radio Galaxies, 5 Narrow Line Radio
Galaxies, 1 Seyfert I galaxy and 11 galaxies or galaxies in clusters. Over 78%
of the identifications we present here are new. The observations we report were
undertaken as part of our targeted search program to identify a new, large
unbiased sample of BL Lac Objects and we therefore discuss the BL Lac sample
extensively. Unlike many previous surveys, we impose no selection criteria
based on optical morphology, color or broadband spectral energy distribution.
Our classifications are based solely on a carefully defined set of
self-consistent spectroscopic classification criteria. We show the 53 RGB
presented here exhibit transitional properties between normal galaxies and BL
Lacs discovered previously. We show there is no clear separation in CaII break
strength between RGB BL Lacs and galaxies, with the distribution of break
strengths varying smoothly between 0% and 50%. We also show that the newly
discovered RGB BL Lacs reside in a "zone of avoidance" in the log(S_x/S_r) vs.
log(S_o/S_r) diagram. This has important implications for BL Lac search
strategies since it shows that RASS BL Lac samples will be severely incomplete
if candidates are chosen only from among those objects with the highest S_x/S_r
flux ratios.Comment: 21 pages text, 189 Figures, 4 tables, LaTeX2E, 4.2MB tar file
(compressed); special style file paper.sty provide
High-frequency radio observations of the Kuehr sample and the epoch-dependent luminosity function of the flat-spectrum quasars
We discuss our ATCA 18.5 and 22 GHz flux density measurements of the Southern
extragalactic sources in the complete 5-GHz sample of Kuehr et al. (1981). The
high-frequency (5-18.5 GHz) spectral indices of steep-spectrum sources for
which we have 18.5-GHz data (66% of the complete sample) are systematically
steeper than the low-frequency (2.7-5 GHz) ones, and there is evidence of an
anti-correlation of high-frequency spectral index with luminosity. The
completeness of the 18.5-GHz data is much higher (89%) for flat-spectrum
sources (mostly quasars), which also exhibit a spectral steepening. Taking
advantage of the almost complete redshift information on flat-spectrum quasars,
we have estimated their 5-GHz luminosity function in several redshift bins. The
results confirm that their radio luminosity density peaks at z_peak \simeq 2.5
but do not provide evidence for deviations from pure luminosity evolution as
hinted at by other data sets. A comparison of our 22-GHz flux density with WMAP
K-band data for flat-spectrum sources suggests that WMAP flux densities may be
low by a median factor of \simeq 1.2. The extrapolations of 5-GHz counts and
luminosity functions of flat-spectrum radio quasars using the observed
distribution of the 5-18.5 GHz spectral indices match those derived directly
from WMAP data, indicating that the high-frequency WMAP survey does not detect
any large population of FSRQs with anomalous spectra.Comment: 11 pages, 5 figures, 1 table, accepted on A&
A ROSAT HRI survey of bright nearby galaxies
We use the extensive public archive of ROSAT High Resolution Imager (HRI)
observations to carry out a statistical investigation of the X-ray properties
of nearby galaxies. Specifically we focus on the sample of 486 bright (B_T <
12.5) northern galaxies studied by Ho, Filippenko and Sargent (HFS) in the
context of their exploration of the optical spectroscopic properties of nearby
galactic nuclei. Over 20% of HFS galaxies are encompassed in ROSAT HRI fields
of reasonable (> 10ks) exposure. The X-ray sources detected within the optical
extent of each galaxy are categorised as either nuclear or non-nuclear
depending on whether the source is positioned within or outside of a 25
arcsecond radius circle centred on the optical nucleus. A nuclear X-ray source
is detected in over 70% of the galaxies harbouring either a Seyfert or LINER
nucleus compared to a detection rate of only ~40% in less active systems. The
correlation of the H alpha luminosity with nuclear X-ray luminosity previously
observed in QSOs and bright Seyfert 1 galaxies appears to extend down into the
regime of ultra-low luminosity (L(x)~10^38 - 10^40 erg/s) active galactic
nuclei (AGN). The inferred accretion rates for this sample of low-luminosity
AGN are significantly sub-Eddington. In total 142 non-nuclear sources were
detected. In combination with published data for M31 this leads to a luminosity
distribution (normalised to an optical blue luminosity of L(B) = 10^10
L(solar)) for the discrete X-ray source population in spiral galaxies of the
form dN/dL38 = 1.0 +/- 0.2 L38^-1.8, where L38 is the X-ray luminosity in units
of 10^38 erg/s. The implied L(x)/L(B) ratio is ~1.1 x 10^39 erg/s/(10^10
L(solar)). The nature of the substantial number of ``super-luminous''
non-nuclear objects detected in the survey is discussed.Comment: 20 pages, 7 figures, accepted for publication in MNRAS. Also
available from http://www.star.le.ac.uk/~tro/papers/xhfs.p
The Hubble Space Telescope Treasury Program on the Orion Nebula Cluster
The Hubble Space Telescope (HST) Treasury Program on the Orion Nebula Cluster
has used 104 orbits of HST time to image the Great Orion Nebula region with the
Advanced Camera for Surveys (ACS), the Wide-Field/Planetary Camera 2 (WFPC2)
and the Near Infrared Camera and Multi Object Spectrograph (NICMOS) instruments
in 11 filters ranging from the U-band to the H-band equivalent of HST. The
program has been intended to perform the definitive study of the stellar
component of the ONC at visible wavelengths, addressing key questions like the
cluster IMF, age spread, mass accretion, binarity and cirumstellar disk
evolution. The scanning pattern allowed to cover a contiguous field of
approximately 600 square arcminutes with both ACS and WFPC2, with a typical
exposure time of approximately 11 minutes per ACS filter, corresponding to a
point source depth AB(F435W) = 25.8 and AB(F775W)=25.2 with 0.2 magnitudes of
photometric error. We describe the observations, data reduction and data
products, including images, source catalogs and tools for quick look preview.
In particular, we provide ACS photometry for 3399 stars, most of them detected
at multiple epochs, WFPC2 photometry for 1643 stars, 1021 of them detected in
the U-band, and NICMOS JH photometry for 2116 stars. We summarize the early
science results that have been presented in a number of papers. The final set
of images and the photometric catalogs are publicly available through the
archive as High Level Science Products at the STScI Multimission Archive hosted
by the Space Telescope Science Institute.Comment: Accepted for publication on the Astrophysical Journal Supplement
Series, March 27, 201
Star Formation Activity in the Galactic HII Complex S255-S257
We present results on the star-formation activity of an optically obscured
region containing an embedded cluster (S255-IR) and molecular gas between two
evolved HII regions S255 and S257. We have studied the complex using optical,
near-infrared (NIR) imaging, optical spectroscopy and radio continnum mapping
at 15 GHz, along with Spitzer-IRAC results. It is found that the main exciting
sources of the evolved HII regions S255 and S257 and the compact HII regions
associated with S255-IR are of O9.5 - B3 V nature, consistent with previous
observations. Our NIR observations reveal 109 likely young stellar object (YSO)
candidates in an area of ~ 4'.9 x 4'.9 centered on S255-IR, which include 69
new YSO candidates. Our observations increased the number of previously
identified YSOs in this region by 32%. To see the global star formation, we
constructed the V-I/V diagram for 51 optically identified IRAC YSOs in an area
of ~ 13' x 13' centered on S255-IR. We suggest that these YSOs have an
approximate age between 0.1 - 4 Myr, indicating a non-coeval star formation.
Using spectral energy distribution models, we constrained physical properties
and evolutionary status of 31 and 16 YSO candidates outside and inside the gas
ridge, respectively. The models suggest that the sources associated within the
gas ridge are of younger population (mean age ~ 1.2 Myr) than the sources
outside the gas ridge (mean age ~ 2.5 Myr). The positions of the young sources
inside the gas ridge at the interface of the HII regions S255 and S257, favor a
site of induced star formation.Comment: 46 pages, 14 figures, 5 tables. Accepted for publication in The
Astrophysical Journa
High-frequency polarization properties of southern Kuhr sources
We have carried out observations at 18.5 GHz with the Australia Telescope
Compact Array (ATCA) of 250 out of the 258 Southern extragalactic sources in
the complete 5 GHz 1 Jy sample by K\"uhr et al. (1981). In this paper we focus
on the polarization properties of this sample, while other properties will be
addressed in a future paper. In our analysis we subdivide the sample into flat
and steep spectrum sources following Stickel et al. (1994) classification,
where spectral indices were measured between 2.7 and 5 GHz. The polarized flux
has been measured with a S/N > 5 for 170 sources (114 flat-spectrum and 56
steep-spectrum) and upper limits have been set for additional 27 sources (12
flat-spectrum and 15 steep-spectrum). The median polarization degree at 18.5
GHz for the flat-spectrum sub-sample is \Pi_{18.5}\simeq 2.7%, about a factor
of 2 higher than at 1.4 GHz (\Pi_{1.4}\simeq 1.4%, based on NVSS data). For
flat-spectrum sources we find a weak correlation between \Pi_{18.5} and the
high frequency (5--18.5 GHz) spectral index. No evidences of significant
correlations of the polarization degree with other source properties are found.
The median value of \Pi_{18.5} for the steep spectrum sources is \simeq 4.8%,
but our sample might be biased against extended sources. We find indications of
a correlation between \Pi_{18.5} and both the low frequency (1.4--5 GHz) and
the high frequency (5--18.5 GHz) spectral indices. An important application of
this work is the possibility to estimate the contamination of CMB polarization
maps by extragalactic radio sources. Our results indicate that such
contamination is within the range of estimates given by Mesa et al. (2002).Comment: 11 pages, 8 Postscripts figures, 6 tables, accepted for publication
on A&A, minor typos correcte
An X-ray Imaging Study of the Stellar Population in RCW49
We present the results of a high-resolution X-ray imaging study of the
stellar population in the Galactic massive star-forming region RCW49 and its
central OB association Westerlund 2. We obtained a 40 ks X-ray image of a
17'x17' field using the Chandra X-ray Observatory and deep NIR images using the
Infrared Survey Facility in a concentric 8'3x8'3 region. We detected 468 X-ray
sources and identified optical, NIR, and Spitzer Space Telescope MIR
counterparts for 379 of them. The unprecedented spatial resolution and
sensitivity of the X-ray image, enhanced by optical and infrared imaging data,
yielded the following results: (1) The central OB association Westerlund 2 is
resolved for the first time in the X-ray band. X-ray emission is detected from
all spectroscopically-identified early-type stars in this region. (2) Most
(86%) X-ray sources with optical or infrared identifications are cluster
members in comparison with a control field in the Galactic Plane. (3) A loose
constraint (2--5 kpc) for the distance to RCW49 is derived from the mean X-ray
luminosity of T Tauri stars. (4) The cluster X-ray population consists of
low-mass pre--main-sequence and early-type stars as obtained from X-ray and NIR
photometry. About 30 new OB star candidates are identified. (5) We estimate a
cluster radius of 6'--7' based on the X-ray surface number density profiles.
(6) A large fraction (90%) of cluster members are identified individually using
complimentary X-ray and MIR excess emission. (7) The brightest five X-ray
sources, two Wolf-Rayet stars and three O stars, have hard thermal spectra.Comment: 19 pages, 17 figures, 4 tables. ApJ in pres
- …