636 research outputs found
Evidence for the Large-Scale Dissociation of Molecular Gas in the Inner Spiral Arms of M81
We compare the detailed distributions of HI, H alpha, and 150 nm far-UV
continuum emission in the spiral arms of M81 at a resolution of 9" (linear
resolution 150 pc at 3.7 Mpc distance). The bright H alpha emission peaks are
always associated with peaks in the far-UV emission. The converse is not always
true; there are many regions of far-UV emission with little corresponding H
alpha. The HI and the far-UV are always closely associated, in the sense that
the HI is often brightest around the edges of the far-UV emission. The effects
of extinction on the morphology are small, even in the far-UV. Extensive far-UV
emission, often with little corresponding H alpha, indicates the presence of
many ``B-stars'', which produce mostly non-ionizing UV photons. These far-UV
photons dissociate a small fraction of an extensive layer of H_2 into HI. The
observed morphology can be understood if ``chimneys'' are common in the spiral
arms of M81, where holes are blown out of the galactic disk, exposing the
bright HII regions and the corresponding far-UV associated with vigorous star
formation. These ``naked'' star-forming regions show little obscuration. H_2 is
turned into HI by UV photons impinging on the interior surfaces of these
chimneys. The intensity of the far-UV radiation measured by UIT can dissociate
the underlying H_2 with a typical density of ~10 H nucleii cm**-3 to produce
the observed amount of HI in the spiral arms of M81. Except for thin surface
layers locally heated in these photo-dissociation regions close to the far-UV
sources, the bulk of the molecular gas in the inner disk of M81 is apparently
too cold to produce much 12CO(1-0) emission.Comment: 12 pages, Latex. 8 postscript files. Better quality versions of the
figures available from ftp://star.herts.ac.uk/pub/Knapen/m81uv . Accepted,
Ap
INTEGRAL detection of hard X-rays from NGC 6334: Nonthermal emission from colliding winds or an AGN?
We report the detection of hard X-ray emission from the field of the
star-forming region NGC 6334 with the the International Gamma-Ray Astrophysics
Laboratory INTEGRAL. The JEM-X monitor and ISGRI imager aboard INTEGRAL and
Chandra ACIS imager were used to construct 3-80 keV images and spectra of NGC
6334. The 3-10 keV and 10-35 keV images made with JEM-X show a complex
structure of extended emission from NGC 6334. The ISGRI source detected in the
energy ranges 20-40 keV and 40-80 keV coincides with the NGC 6334 ridge. The
20-60 keV flux from the source is (1.8+-0.37)*10(-11) erg cm(-2) s(-1).
Spectral analysis of the source revealed a hard power-law component with a
photon index about 1. The observed X-ray fluxes are in agreement with
extrapolations of X-ray imaging observations of NGC 6334 by Chandra ACIS and
ASCA GIS. The X-ray data are consistent with two very different physical
models. A probable scenario is emission from a heavily absorbed, compact and
hard Chandra source that is associated with the AGN candidate radio source NGC
6334B. Another possible model is the extended Chandra source of nonthermal
emission from NGC 6334 that can also account for the hard X-ray emission
observed by INTEGRAL. The origin of the emission in this scenario is due to
electron acceleration in energetic outflows from massive early type stars. The
possibility of emission from a young supernova remnant, as suggested by earlier
infrared observations of NGC 6334, is constrained by the non-detection of 44Ti
lines.Comment: 8 pages, 8 figures, Astronomy and Astrophysics (in press
Three ways to solve the orbit of KIC11558725: a 10 day beaming sdB+WD binary with a pulsating subdwarf
The recently discovered subdwarf B (sdB) pulsator KIC11558725 features a rich
g-mode frequency spectrum, with a few low-amplitude p-modes at short periods,
and is a promising target for a seismic study aiming to constrain the internal
structure of this star, and of sdB stars in general.
We have obtained ground-based spectroscopic Balmer-line radial-velocity
measurements of KIC11558725, spanning the 2010 and 2011 observing seasons. From
these data we have discovered that KIC11558725 is a binary with period P=10.05
d, and that the radial-velocity amplitude of the sdB star is 58 km/s.
Consequently the companion of the sdB star has a minimum mass of 0.63 M\odot,
and is therefore most likely an unseen white dwarf.
We analyse the near-continuous 2010-2011 Kepler light curve to reveal orbital
Doppler-beaming light variations at the 238 ppm level, which is consistent with
the observed spectroscopic orbital radial-velocity amplitude of the subdwarf.
We use the strongest 70 pulsation frequencies in the Kepler light curve of the
subdwarf as clocks to derive a third consistent measurement of the orbital
radial-velocity amplitude, from the orbital light-travel delay.
We use our high signal-to-noise average spectra to study the atmospheric
parameters of the sdB star, deriving Teff = 27 910K and log g = 5.41 dex, and
find that carbon, nitrogen and oxygen are underabundant relative to the solar
mixture.
Furthermore, we extract more than 160 significant frequencies from the Kepler
light curve. We investigate the pulsation frequencies for expected period
spacings and rotational splittings. We find period-spacing sequences of
spherical-harmonic degrees \ell=1 and \ell=2, and we associate a large fraction
of the g-modes in KIC11558725 with these sequences. From frequency splittings
we conclude that the subdwarf is rotating subsynchronously with respect to the
orbit
Cyg X-3: Not seen in high-energy gamma rays by COS-B
COS-B had Cyg X-3 within its field of view during 7 observation periods between 1975 and 1982 for in total approximately 300 days. In the skymaps (70 meV E 5000 meV) of the Cyg-X region produced for each of these observations and in the summed map, a broad complex structure is visible in the region 72 deg approximately less than 1 approximately less than 85 deg, approximately less than 5 deg. No resolved source structure is visible at the position of Cyg X-3, but a weak signal from Cyg X-3 could be hidden in the structured gamma-ray background. Therefore, the data has been searched for a 4.8 h timing signature, as well as for a source signal in the sky map in addition to the diffuse background structure as estimated from tracers of atomic and molecular gas
The orbits of subdwarf B + main-sequence binaries. I: The sdB+G0 system PG 1104+243
The predicted orbital period histogram of an sdB population is bimodal with a
peak at short ( 250 days) periods. Observationally, there
are many short-period sdB systems known, but only very few long-period sdB
binaries are identified. As these predictions are based on poorly understood
binary interaction processes, it is of prime importance to confront the
predictions to observational data. In this contribution we aim to determine the
absolute dimensions of the long-period sdB+MS binary system PG1104+243.
High-resolution spectroscopy time-series were obtained with HERMES at the
Mercator telescope at La Palma, and analyzed to obtain radial velocities of
both components. Photometry from the literature was used to construct the
spectral energy distribution (SED) of the binary. Atmosphere models were used
to fit this SED and determine the surface gravity and temperature of both
components. The gravitational redshift provided an independent confirmation of
the surface gravity of the sdB component. An orbital period of 753 +- 3 d and a
mass ratio of q = 0.637 +- 0.015 were found from the RV-curves. The sdB
component has an effective temperature of Teff = 33500 +- 1200 K and a surface
gravity of logg = 5.84 +- 0.08 dex, while the cool companion is found to be a
G-type star with Teff = 5930 +- 160 K and logg = 4.29 +- 0.05 dex. Assuming a
canonical mass of Msdb = 0.47 Msun, the MS component has a mass of 0.74 +- 0.07
Msun, and its Teff corresponds to what is expected for a terminal age
main-sequence star with sub-solar metalicity. PG1104+243 is the first
long-period sdB binary in which accurate physical parameters of both components
could be determined, and the first sdB binary in which the gravitational
redshift is measured. Furthermore, PG1104+243 is the first sdB+MS system that
shows consistent evidence for being formed through stable Roche-lobe overflow.Comment: Accepted by A&A on 05-10-201
Multifrequency Observations of the Virgo Blazars 3C 273 and 3C 279 in CGRO Cycle 8
We report first observational results of multifrequency campaigns on the
prominent Virgo blazars 3C 273 and 3C 279 which were carried out in January and
February 1999. Both blazars are detected from radio to gamma-ray energies. We
present the measured X- to gamma-ray spectra of both sources, and for 3C 279 we
compare the 1999 broad-band (radio to gamma-ray) spectrum to measured previous
ones.Comment: 5 pages including 3 figures, latex2e, to appear in: 'Proc. of the 5th
Compton Symposium', AIP, in pres
Constraints on the galactic distribution of cosmic rays from the COS-B gamma-ray data
The diffuse component of the galactic high energy gamma rays results mainly from the interaction of CR nuclei and electrons with the nuclei of the interstellar gas. An additional contribution is obtained from the interaction of CR electrons with the interstellar photons through the inverse-Compton (IC) process. Gamma ray astronomy therefore offers an excellent means to study the distribution of CR particles throughout the Galaxy, but it is essential to know the distribution of the target interstellar gas particles, the major constituents being atomic and molecular hydrogen. Large scale millimeter wave surveys of the CO molecule covering more than half of the Milky Way, obtained with the Columbia 1.2 m telescopes, are currently available and are used to trace the H2; the COS-B observations have sufficient resolution and sensitivity to constrain the relation between the integrated CO line intensity and the molecular hydrogen column density
Diffuse continuum gamma rays from the Galaxy
A new study of the diffuse Galactic gamma-ray continuum radiation is
presented, using a cosmic-ray propagation model which includes nucleons,
antiprotons, electrons, positrons, and synchrotron radiation. Our treatment of
the inverse Compton (IC) scattering includes the effect of anisotropic
scattering in the Galactic interstellar radiation field (ISRF) and a new
evaluation of the ISRF itself. Models based on locally measured electron and
nucleon spectra and synchrotron constraints are consistent with gamma-ray
measurements in the 30-500 MeV range, but outside this range excesses are
apparent. A harder nucleon spectrum is considered but fitting to gamma rays
causes it to violate limits from positrons and antiprotons. A harder
interstellar electron spectrum allows the gamma-ray spectrum to be fitted above
1 GeV as well, and this can be further improved when combined with a modified
nucleon spectrum which still respects the limits imposed by antiprotons and
positrons. A large electron/IC halo is proposed which reproduces well the
high-latitude variation of gamma-ray emission. The halo contribution of
Galactic emission to the high-latitude gamma-ray intensity is large, with
implications for the study of the diffuse extragalactic component and
signatures of dark matter. The constraints provided by the radio synchrotron
spectral index do not allow all of the <30 MeV gamma-ray emission to be
explained in terms of a steep electron spectrum unless this takes the form of a
sharp upturn below 200 MeV. This leads us to prefer a source population as the
origin of the excess low-energy gamma rays.Comment: Final version accepted for publication in The Astrophysical Journal
(vol. 537, July 10, 2000 issue); Many Updates; 20 pages including 49
ps-figures, uses emulateapj.sty. More details can be found at
http://www.gamma.mpe-garching.mpg.de/~aws/aws.htm
Light Element Evolution and Cosmic Ray Energetics
Using cosmic-ray energetics as a discriminator, we investigate evolutionary
models of LiBeB. We employ a Monte Carlo code which incorporates the delayed
mixing into the ISM both of the synthesized Fe, due to its incorporation into
high velocity dust grains, and of the cosmic-ray produced LiBeB, due to the
transport of the cosmic rays. We normalize the LiBeB production to the integral
energy imparted to cosmic rays per supernova. Models in which the cosmic rays
are accelerated mainly out of the average ISM significantly under predict the
measured Be abundance of the early Galaxy, the increase in [O/Fe] with
decreasing [Fe/H] notwithstanding. We suggest that this increase could be due
to the delayed mixing of the Fe. But, if the cosmic-ray metals are accelerated
out of supernova ejecta enriched superbubbles, the measured Be abundances are
consistent with a cosmic-ray acceleration efficiency that is in very good
agreement with the current epoch data. We also find that neither the above
cosmic-ray origin models nor a model employing low energy cosmic rays
originating from the supernovae of only very massive progenitors can account
for the Li data at values of [Fe/H] below 2.Comment: latex 19 pages, 2 tables, 10 eps figures, uses aastex.cls natbib.sty
Submitted to the Astrophysical Journa
On the nature of the hard X-ray source IGR J2018+4043
We found a very likely counterpart to the recently discovered hard X-ray
source IGR J2018+4043 in the multi-wavelength observations of the source field.
The source, originally discovered in the 20-40 keV band, is now confidently
detected also in the 40-80 keV band, with a flux of (1.4 +/- 0.4) x 10(-11) erg
cm(-2) s(-1). A 5 ks Swift observation of the IGR J2018+4043 field revealed a
hard point-like source with the observed 0.5-10 keV flux of 3.4(+0.7)(-0.8) x
10(-12) erg cm(-2) s(-1) (90% confidence level) at alpha = 20h18m38.55s, delta
= +40d41m00.4s (with a 4.2" uncertainty). The combined Swift-INTEGRAL spectrum
can be described by an absorbed power-law model with photon index gamma = 1.3
+/- 0.2 and N_H = 6.1(+3.2)(-2.2) x 10(22) cm(-2). In archival optical and
infrared data we found a slightly extended and highly absorbed object at the
Swift source position. There is also an extended VLA 1.4 GHz source peaked at a
beam-width distance from the optical and X-ray positions. The observed
morphology and multiwavelength spectra of IGR J2018+4043 are consistent with
those expected for an obscured accreting object, i.e. an AGN or a Galactic
X-ray binary. The identification suggests possible connection of IGR J2018+4043
to the bright gamma-ray source GEV J2020+4023 (3EG J2020+4017) detected by COS
B and CGRO EGRET in the gamma-Cygni SNR field.Comment: 5 pages, 3 figures, uses emulateapj styl
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