204 research outputs found
Design and status of the detector block for the ISO-SWS
The Short Wave Spectrometer (SWS) is one of the two spectrometers for the Infrared Space Observatory (ISO). It consists of a pair of grating spectrometers and a Fabry-Perot interferometer. Together, the grating spectrometers cover the wavelength range 2.4 to 45 microns, at a resolution between 1000 and 2000. The Fabry-Perot interferometer, in series with one of the grating spectrometers, provides a resolution of about 20,000 at the wavelengths between 15 and 35 microns. The SWS is being built by the Space Research Organization of the Netherlands and the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. The spectrometer has 52 discrete detectors, most of which are bulk detectors. In the design of the spectrometer, the main emphasis is on the sensitivity of the individual channels, rather than on the number of detectors. This was one of the main reasons to select non-destructive read-out circuits, with a separate heated-JFET pre-amplifier for each individual detector. The signals are amplified and filtered in parallel. The engineering tests on the SWS detector block have not yet been completed. The design of the detector block is described and the present problem areas are indicated
Past and present star formation in the SMC: NGC 346 and its neighborhood
In the quest of understanding how star formation occurs and propagates in the
low metallicity environment of the Small Magellanic Cloud (SMC), we acquired
deep F555W (~V), and F814W (~I) HST/ACS images of the young and massive star
forming region NGC 346. These images and their photometric analysis provide us
with a snapshot of the star formation history of the region. We find evidence
for star formation extending from ~10 Gyr in the past until ~150 Myr in the
field of the SMC. The youngest stellar population (~3 +/- 1 Myr) is associated
with the NGC 346 cluster. It includes a rich component of low mass pre-main
sequence stars mainly concentrated in a number of sub-clusters, spatially co-
located with CO clumps previously detected by Rubio et al. (2000). Within our
analysis uncertainties, these sub-clusters appear coeval with each other. The
most massive stars appear concentrated in the central sub-clusters, indicating
possible mass segregation. A number of embedded clusters are also observed.
This finding, combined with the overall wealth of dust and gas, could imply
that star formation is still active. An intermediate age star cluster, BS90,
formed ~4.3 +/-0.1 Gyr ago, is also present in the region. Thus, this region of
the SMC has supported star formation with varying levels of intensity over much
of the cosmic time.Comment: 38 pages, 13 figures, 3 tables; AJ accepte
The Fourth Positive System of Carbon Monoxide in the Hubble Space Telescope Spectra of Comets
The rich structure of the Fourth Positive System (A-X) of carbon monoxide
accounts for many of the spectral features seen in long slit HST-STIS
observations of comets 153P/Ikeya-Zhang, C/2001 Q4 (NEAT), and C/2000 WM1
(LINEAR), as well as in the HST-GHRS spectrum of comet C/1996 B2 Hyakutake. A
detailed CO fluorescence model is developed to derive the CO abundances in
these comets by simultaneously fitting all of the observed A-X bands. The model
includes the latest values for the oscillator strengths and state parameters,
and accounts for optical depth effects due to line overlap and self-absorption.
The model fits yield radial profiles of CO column density that are consistent
with a predominantly native source for all the comets observed by STIS. The
derived CO abundances relative to water in these comets span a wide range, from
0.44% for C/2000 WM1 (LINEAR), 7.2% for 153P/Ikeya-Zhang, 8.8% for C/2001 Q4
(NEAT) to 20.9% for C/1996 B2 (Hyakutake). The subtraction of the CO spectral
features using this model leads to the first identification of a molecular
hydrogen line pumped by solar HI Lyman-beta longward of 1200A in the spectrum
of comet 153P/Ikeya-Zhang. (Abridged)Comment: 12 pages, 11 figures, ApJ accepte
STIS UV spectroscopy of early B supergiants in M31
We analyze STIS spectra in the 1150-1700 Angstrom wavelength range obtained
for six early B supergiants in the neighboring galaxy M31. Because of their
likely high (nearly solar) abundance, these stars were originally chosen to be
directly comparable to their Galactic counterparts, and represent a much-needed
addition to our current sample of B-type supergiants, in our efforts to study
the dependence of the Wind Momentum-Luminosity Relationship on spectral type
and metallicity. As a first step to determine wind momenta we fit the P-Cygni
profiles of the resonance lines of N V, Si IV and C IV with standard methods,
and derive terminal velocities for all of the STIS targets. From these lines we
also derive ionic stellar wind column densities. Our results are compared with
those obtained previously in Galactic supergiants, and confirm earlier claims
of `normal' wind line intensities and terminal velocities in M31. For half of
the sample we find evidence for an enhanced maximum turbulent velocity when
compared to Galactic counterparts.Comment: 15 pages, 9 figures, 2 tables. Accepted for publication in The
Astrophysical Journa
Terminal velocities of luminous, early-type SMC stars
Ultraviolet spectra from the Space Telescope Imaging Spectrograph (STIS) are
used to determine terminal velocities for 11 O and B-type giants and
supergiants in the Small Magellanic Cloud (SMC) from the Si IV and C IV
resonance lines. Using archival data from observations with the Goddard
High-Resolution Spectrograph and the International Ultraviolet Explorer
telescope, terminal velocities are obtained for a further five B-type
supergiants. We discuss the metallicity dependence of stellar terminal
velocities, finding no evidence for a significant scaling between Galactic and
SMC metallicities for Teff < 30,000 K, consistent with the predictions of
radiation driven wind theory for supergiant stars. A comparison of the
ratio between the SMC and Galactic samples, while
consistent with the above statement, emphasizes that the uncertainties in the
distances to galactic O-stars are a serious obstacle to a detailed comparison
with theory. For the SMC sample there is considerable scatter in this ratio at
a given effective temperature, perhaps indicative of uncertainties in stellar
masses.Comment: 28 pages, 8 figures, accepted by ApJ; minor revisions prior to
acceptanc
Understanding B-type Supergiants in the Low Metallicity Environment of the SMC
Spectroscopic analyses of 7 SMC B-type supergiants and 1 giant have been
undertaken using high resolution optical data obtained on the VLT with UVES.
FASTWIND, a non-LTE, spherical,line-blanketed model atmosphere code was used to
derive atmospheric and wind parameters of these stars as well as their absolute
abundances. The implications of these results for stellar evolution and line
driven wind theory are discussed.Comment: 19 pages, 6 tables, 10 figures, accepted for publication in Astronomy
and Astrophysics (5/12/2003
Spatial distribution of low-energy plasma around 2 comet 67P/CG from Rosetta measurements
International audienceWe use measurements from the Rosetta plasma consortium (RPC) Langmuir probe (LAP) and mutual impedance probe (MIP) to study the spatial distribution of low-energy plasma in the near-nucleus coma of comet 67P/Churyumov-Gerasimenko. The spatial distribution is highly structured with the highest density in the summer hemisphere and above the region connecting the two main lobes of the comet, i.e. the neck region. There is a clear correlation with the neutral density and the plasma to neutral density ratio is found to be ∼1-2·10 −6 , at a cometocentric distance of 10 km and at 3.1 AU from the sun. A clear 6.2 h modulation of the plasma is seen as the neck is exposed twice per rotation. The electron density of the collisonless plasma within 260 km from the nucleus falls of with radial distance as ∼1/r. The spatial structure indicates that local ionization of neutral gas is the dominant source of low-energy plasma around the comet
Stellar spectroscopy far beyond the Local Group
Multi-object spectroscopic observations of blue supergiants in NGC 3621, a
spiral galaxy at a distance of 6.7 Mpc, carried out with the ESO VLT and FORS
are presented. We demonstrate the feasibility of quantitative stellar
spectroscopy at distances approaching a ten-fold increase over previous
investigations by determining chemical composition, stellar parameters,
reddening, extinction and wind properties of one of our targets, a supergiant
of spectral type A1 Ia located in the outskirts of NGC 3621. The metallicity
(determined from iron group elements) is reduced by a factor of two relative to
the sun in qualitative agreement with results from previous abundance studies
based on H II region oxygen emission lines. Reddening and extinction are E(B-V)
= 0.12 and Av = 0.37, respectively, mostly caused by the galactic foreground.
Comparing stellar wind momentum and absolute V magnitude with galactic and M31
counterparts we confirm the potential of the wind momentum-luminosity
relationship as an alternative tool to estimate extragalactic distances.Comment: 9 pages, 4 figures, 1 table, accepted for publication in The
Astrophysical Journal Letter
Modelling the orbital modulation of ultraviolet resonance lines in high-mass X-ray binaries
The stellar-wind structure in high-mass X-ray binaries (HMXBs) is
investigated through modelling of their ultraviolet (UV) resonance lines. For
the OB supergiants in two systems, Vela X-1 and 4U1700-37, high-resolution UV
spectra are available; for Cyg X-1, SMC X-1, and LMC X-4 low-resolution spectra
are used. In order to account for the non-monotonic velocity structure of the
stellar wind, a modified version of the Sobolev Exact Integration (SEI) method
by Lamers et al. (1987) is applied. The orbital modulation of the UV resonance
lines provides information on the size of the Stroemgren zone surrounding the
X-ray source. The amplitude of the observed orbital modulation (known as the
Hatchett-McCray effect), however, also depends on the density- and velocity
structure of the ambient wind. Model profiles are presented that illustrate the
effect on the appearance of the HM effect by varying stellar-wind parameters.
The q parameter of Hatchett & McCray (1977), as well as other parameters
describing the supergiant's wind structure, are derived for the 5 systems. The
X-ray luminosity needed to create the observed size of the Stroemgren zone is
consistent with the observed X-ray flux. The derived wind parameters are
compared to those determined in single OB supergiants of similar spectral type.
Our models naturally explain the observed absence of the HM effect in
4U1700-37. The orbital modulation in Vela X-1 indicates that besides the
Stroemgren zone other structures are present in the stellar wind (such as a
photo-ionization wake). The ratio of the wind velocity and the escape velocity
is found to be lower in OB supergiants in HMXBs than in single OB supergiants
of the same effective temperature.Comment: 29 pages, good quality figures 11, 12, 13, A2 & B1 available upon
request from JvL. Accepted for publication in Astronomy & Astrophysic
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