3,382 research outputs found
Orbit of the Mercury-Manganese binary 41 Eridani
Context. Mercury-manganese (HgMn) stars are a class of slowly rotating
chemically peculiar main-sequence late B-type stars. More than two-thirds of
the HgMn stars are known to belong to spectroscopic binaries.
Aims. By determining orbital solutions for binary HgMn stars, we will be able
to obtain the masses for both components and the distance to the system.
Consequently, we can establish the position of both components in the
Hertzsprung-Russell diagram and confront the chemical peculiarities of the HgMn
stars with their age and evolutionary history.
Methods. We initiated a program to identify interferometric binaries in a
sample of HgMn stars, using the PIONIER near-infrared interferometer at the
VLTI on Cerro Paranal, Chile. For the detected systems, we intend to obtain
full orbital solutions in conjunction with spectroscopic data.
Results. The data obtained for the SB2 system 41 Eridani allowed the
determination of the orbital elements with a period of just five days and a
semi-major axis of under 2 mas. Including published radial velocity
measurements, we derived almost identical masses of 3.17 +/- 0.07 M_Sun for the
primary and 3.07 +/- 0.07 M_Sun for the secondary. The measured magnitude
difference is less than 0.1 mag. The orbital parallax is 18.05 +/- 0.17 mas,
which is in good agreement with the Hipparcos trigonometric parallax of 18.33
+/- 0.15 mas. The stellar diameters are resolved as well at 0.39 +/- 0.03 mas.
The spin rate is synchronized with the orbital rate.Comment: 5 pages, 4 figure
Search of X-ray emission from roAp stars: The case of gamma Equulei
The detection of X-ray emission from Ap stars can be an indicator for the
presence of magnetic activity and dynamo action, provided different origins for
the emission, such as wind shocks and close late-type companions, can be
excluded. Here we report on results for gamma Equu, the only roAp star for
which an X-ray detection is reported in ROSAT catalogs. We use high resolution
imaging in X-rays with Chandra and in the near-infrared with NACO/VLT that
allow us to spatially resolve companions down to ~1" and ~0.06" separations,
respectively. The bulk of the X-ray emission is associated with a companion of
gamma Equu identified in our NACO image. Assuming coevality with the primary
roAp star (~900 Myr), the available photometry for the companion points at a
K-type star with ~0.6 M_sun. Its X-ray properties are in agreement with the
predictions for its age and mass. An excess of photons with respect to the
expected background and contribution from the nearby companion is observed near
the optical position of gamma Equu. We estimate an X-ray luminosity of log L_x
[erg/s] = 26.6 and log(L_x/L_bol) = -7.9 for this emission. A small offset
between the optical and the X-ray image leaves some doubt on its association
with the roAp star. The faint X-ray emission that we tentatively ascribe to the
roAp star is difficult to explain as a solar-like stellar corona due to its
very low L_x/L_bol level and the very long rotation period of gamma Equu. It
could be produced in magnetically confined wind shocks implying a mass loss
rate of ~10^(-14) M_sun/yr or from an additional unknown late-type companion at
separation ~0.4". If confirmed by future deeper X-ray observations this
emission could point at the origin for the presence of radioactive elements on
some roAp stars.Comment: Accepted for publication in Astronomy & Astrophysics (5 pages
Intranasal sodium citrate solution improves olfaction in post-viral hyposmia
Background: Calcium plays an integral role in olfactory signal transduction, including feedback inhibition. Sodium citrate acts as a calcium sequestrant and when applied intranasally, reduces free calcium available for feedback inhibition, which should theoretically improve olfaction. We aimed to investigate the utility of intranasal sodium citrate in improving the olfactory function of hyposmic patients, by performing this prospective placebo controlled, single-blinded trial. Methodology: Monorhinal olfactory testing for odour identification and threshold was performed in hyposmic patients using “Sniffin’ Sticks”, before and after treatment. Treatment consisted of sodium citrate solution application to the olfactory cleft. Sodium chloride solution was applied to the contralateral olfactory cleft, which therefore acted as placebo control. Patients were blinded to the side of sodium citrate application, and side of treatment was randomized between patients. Results: 57 patients participated in the trial, aged 22-79. Causes of hyposmia included: post-viral (7); posttraumatic (10); sinonasal (30) and idiopathic (10). Compared with placebo, there was significant improvement in the identification scores of participants with post-viral hyposmia, following sodium citrate treatment. No significant change in olfactory function occurred for either identification or threshold in any other aetiological subgroup. Conclusions: Intranasal sodium citrate may be of benefit to patients with post-viral hyposmia
Massive envelopes and filaments in the NGC 3603 star forming region
The formation of massive stars and their arrival on the zero-age
main-sequence occurs hidden behind dense clouds of gas and dust. In the giant
Hii region NGC 3603, the radiation of a young cluster of OB stars has dispersed
dust and gas in its vicinity. At a projected distance of 2:5 pc from the
cluster, a bright mid-infrared (mid-IR) source (IRS 9A) had been identified as
a massive young stellar object (MYSO), located on the side of a molecular clump
(MM2) of gas facing the cluster. We investigated the physical conditions in
MM2, based on APEX sub-mm observations using the SABOCA and SHFI instruments,
and archival ATCA 3 mm continuum and CS spectral line data. We resolved MM2
into several compact cores, one of them closely associated with IRS 9A. These
are likely infrared dark clouds as they do not show the typical hot-core
emission lines and are mostly opaque against the mid-IR background. The compact
cores have masses of up to several hundred times the solar mass and gas
temperatures of about 50 K, without evidence of internal ionizing sources. We
speculate that IRS 9A is younger than the cluster stars, but is in an
evolutionary state after that of the compact cores
Jet stability, dynamics and energy transport
Relativistic jets carry energy and particles from compact to very large
scales compared with their initial radius. This is possible due to their
remarkable collimation despite their intrinsic unstable nature. In this
contribution, I review the state-of-the-art of our knowledge on instabilities
growing in those jets and several stabilising mechanisms that may give an
answer to the question of the stability of jets. In particular, during the last
years we have learned that the limit imposed by the speed of light sets a
maximum amplitude to the instabilities, contrary to the case of classical jets.
On top of this stabilising mechanism, the fast growth of unstable modes with
small wavelengths prevents the total disruption and entrainment of jets. I also
review several non-linear processes that can have an effect on the collimation
of extragalactic and microquasar jets. Within those, I remark possible causes
for the decollimation and decelleration of FRI jets, as opposed to the
collimated FRII's. Finally, I give a summary of the main reasons why jets can
propagate through such long distances.Comment: For the proceedings of High Energy Phenomena in Relativistic Outflows
III (HEPRO III, IJMPD, accepted). 12 page
High Latitude Radio Emission in a Sample of Edge-On Spiral Galaxies
We have mapped 16 edge-on galaxies at 20 cm using the VLA. For 5 galaxies, we
could form spectral index, energy and magnetic field maps. We find that all but
one galaxy show evidence for non-thermal high latitude radio continuum
emission, suggesting that cosmic ray halos are common in star forming galaxies.
The high latitude emission is seen over a variety of spatial scales and in
discrete and/or smooth features. In general, the discrete features emanate from
the disk, but estimates of CR diffusion lengths suggest that diffusion alone is
insufficient to transport the particles to the high latitudes seen (> 15 kpc in
one case). Thus CRs likely diffuse through low density regions and/or are
assisted by other mechanisms (e.g. winds). We searched for correlations between
the prevalence of high latitude radio emission and a number of other
properties, including the global SFR, supernova input rate per unit star
forming, and do not find clear correlations with any of these properties.Comment: 40 pages of text, 3 figures, 6 tables, and an appendix of 21 jpeg
figures (which is a radio continuum catalogue of 17 galaxies). to appear in
A. J. (around January 1999
Properties of bow-shock sources at the Galactic center
There are an enigmatic population of massive stars around the Galactic Center
(GC) that were formed some Ma ago. A fraction of these stars has been found to
orbit the supermassive black hole, SgrA*, in a projected clockwise disk, which
suggests that they were formed in a formerly existing dense disk around SgrA*.
We focus on the extended, near-infrared (NIR) sources IRS1W, IRS5, IRS10W, and
IRS21 that have been suggested to be young, massive stars that form bow-shocks
through their interaction with the ISM. Their nature has impeded accurate
determination of their orbital parameters. We aim at establishing their nature
and kinematics to test whether they form part of the clockwise disk. We
performed NIR multi-wavelength imaging using adaptive optics (AO) and sparse
aperture masking (SAM). We introduce a new method for self-calibration of the
SAM PSF in dense stellar fields. The emission mechanism, morphology and
kinematics of the targets were examined via 3D bow-shock models. We confirm
previous findings that IRS21, IRS1W, and IRS5 are bow-shocks created by the
interaction between mass-losing stars and the interstellar gas. The nature of
IRS10W remains unclear. Our modeling shows that the bow-shock-emission is
caused by thermal emission while the scattering of stellar light does not play
any significant role. IRS 1W appears to be a bow-shock produced by an
anisotropic stellar wind or by locally inhomogeneous ISM density. Our best-fit
models provide an estimate of the local proper motion of the ISM in the NA in
agreement with the published models. Assuming that all of the sources are tied
to SgrA*, their orbital planes were obtained via a Monte-Carlo simulation. Our
orbital analysis suggests that they are not part of any of the clockwise disk.
We thus add more evidence to recent findings that a large part of the massive
stars show apparently random orbital orientations.Comment: accepted for publication by A&A, 17 pages, 11 figures, 1 appendi
Unveiling the near-infrared structure of the massive-young stellar object NGC 3603 IRS 9A with sparse aperture masking and spectroastrometry
Contemporary theory holds that massive stars gather mass during their initial
phases via accreting disk-like structures. However, conclusive evidence for
disks has remained elusive for the most massive young objects. This is mainly
due to significant observational challenges. Incisive studies, even targeting
individual objects, are therefore relevant to the progression of the field. NGC
3603 IRS 9A* is a young massive stellar object still surrounded by an envelope
of molecular gas. Previous mid-infrared observations with long-baseline
interferometry provided evidence for a disk of 50 mas diameter at its core.
This work aims at a comprehensive study of the physics and morphology of IRS 9A
at near-infrared wavelengths. New sparse aperture masking interferometry data
taken with NACO/VLT at Ks and Lp filters were obtained and analysed together
with archival CRIRES spectra of the H2 and BrG lines. The calibrated
visibilities recorded at Ks and Lp bands suggest the presence of a partially
resolved compact object of 30 mas at the core of IRS 9A, together with the
presence of over-resolved flux. The spectroastrometric signal of the H2 line
shows that this spectral feature proceeds from the large scale extended
emission (300 mas) of IRS 9A, while the BrG line appears to be formed at the
core of the object (20 mas). This scenario is consistent with the brightness
distribution of the source for near- and mid-infrared wavelengths at various
spatial scales. However, our model suffers from remaining inconsistencies
between SED modelling and the interferometric data. Moreover, the BrG
spectroastrometric signal indicates that the core of IRS 9A exhibits some form
of complexity such as asymmetries in the disk. Future high-resolution
observations are required to confirm the disk/envelope model and to flesh out
the details of the physical form of the inner regions of IRS 9A.Comment: Accepted to be published in Astronomy & Astrophysics, 13 pages, 14
figure
Induced star formation in interacting galaxies
Measurements of H alpha emission line fluxes and FIR fluxes in approx. 100 interacting spirals were used to investigate the effects of close tidal interactions on the disk and nuclear star formation rates in galaxies. Two samples of interacting spirals were studied, a complete sample of close pairs, and a set of strongly perturbed systems from the Arp atlas. Both the integrated H alpha luminosities and FIR luminosities are enhanced in the interacting galaxies, indicating that the encounters indeed trigger massive star formation in many cases. The response of individual galaxies is highly variable, however. A majority of the interacting spirals exhibit normal star formation rates, while a small fraction are undergoing bursts with luminosities which are rarely, if ever, observed in noninteracting systems. Virtually all of the latter are in the Arp sample, indicating that the Arp atlas is heavily biased to the most active star forming systems
Dynamical mass of the O-type supergiant in Zeta Orionis A
A close companion of Zeta Orionis A was found in 2000 with the Navy Precision
Optical Interferometer (NPOI), and shown to be a physical companion. Because
the primary is a supergiant of type O, for which dynamical mass measurements
are very rare, the companion was observed with NPOI over the full 7-year orbit.
Our aim was to determine the dynamical mass of a supergiant that, due to the
physical separation of more than 10 AU between the components, cannot have
undergone mass exchange with the companion. The interferometric observations
allow measuring the relative positions of the binary components and their
relative brightness. The data collected over the full orbital period allows all
seven orbital elements to be determined. In addition to the interferometric
observations, we analyzed archival spectra obtained at the Calar Alto, Haute
Provence, Cerro Armazones, and La Silla observatories, as well as new spectra
obtained at the VLT on Cerro Paranal. In the high-resolution spectra we
identified a few lines that can be associated exclusively to one or the other
component for the measurement of the radial velocities of both. The combination
of astrometry and spectroscopy then yields the stellar masses and the distance
to the binary star. The resulting masses for components Aa of 14.0 solar masses
and Ab of 7.4 solar masses are low compared to theoretical expectations, with a
distance of 294 pc which is smaller than a photometric distance estimate of 387
pc based on the spectral type B0III of the B component. If the latter (because
it is also consistent with the distance to the Orion OB1 association) is
adopted, the mass of the secondary component Ab of 14 solar masses would agree
with classifying a star of type B0.5IV. It is fainter than the primary by about
2.2 magnitudes in the visual. The primary mass is then determined to be 33
solar masses
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