427 research outputs found
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
A close halo of large transparent grains around extreme red giant stars
Intermediate-mass stars end their lives by ejecting the bulk of their
envelope via a slow dense wind back into the interstellar medium, to form the
next generation of stars and planets. Stellar pulsations are thought to elevate
gas to an altitude cool enough for the condensation of dust, which is then
accelerated by radiation pressure from starlight, entraining the gas and
driving the wind. However accounting for the mass loss has been a problem due
to the difficulty in observing tenuous gas and dust tens of milliarcseconds
from the star, and there is accordingly no consensus on the way sufficient
momentum is transferred from the starlight to the outflow. Here, we present
spatially-resolved, multi-wavelength observations of circumstellar dust shells
of three stars on the asymptotic giant branch of the HR diagram. When imaged in
scattered light, dust shells were found at remarkably small radii (<~ 2 stellar
radii) and with unexpectedly large grains (~300 nm radius). This proximity to
the photosphere argues for dust species that are transparent to starlight and
therefore resistant to sublimation by the intense radiation field. While
transparency usually implies insufficient radiative pressure to drive a wind,
the radiation field can accelerate these large grains via photon scattering
rather than absorption - a plausible mass-loss mechanism for lower-amplitude
pulsating stars.Comment: 13 pages, 1 table, 6 figure
The radius and mass of the subgiant star bet Hyi from interferometry and asteroseismology
We have used the Sydney University Stellar Interferometer (SUSI) to measure
the angular diameter of beta Hydri. This star is a nearby G2 subgiant whose
mean density was recently measured with high precision using asteroseismology.
We determine the radius and effective temperature of the star to be
1.814+/-0.017 R_sun (0.9%) and 5872+/-44 K (0.7%) respectively. By combining
this value with the mean density, as estimated from asteroseismology, we make a
direct estimate of the stellar mass. We find a value of 1.07+/-0.03 M_sun
(2.8%), which agrees with published estimates based on fitting in the H-R
diagram, but has much higher precision. These results place valuable
constraints on theoretical models of beta Hyi and its oscillation frequencies.Comment: 3 figures, 3 tables, to appear in MNRAS Letter
Modeling Forbidden Line Emission Profiles from Colliding Wind Binaries
This paper presents calculations for forbidden emission line profile shapes
arising from colliding wind binaries. The main application is for systems
involving a Wolf-Rayet (WR) star and an OB star companion. The WR wind is
assumed to dominate the forbidden line emission. The colliding wind interaction
is treated as an archimedean spiral with an inner boundary. Under the
assumptions of the model, the major findings are as follows. (a) The
redistribution of the WR wind as a result of the wind collision is not flux
conservative but typically produces an excess of line emission; however, this
excess is modest at around the 10% level. (b) Deviations from a flat-top
profile shape for a spherical wind are greatest for viewing inclinations that
are more nearly face-on to the orbital plane. At intermediate viewing
inclinations, profiles display only mild deviations from a flat-top shape. (c)
The profile shape can be used to constrain the colliding wind bow shock opening
angle. (d) Structure in the line profile tends to be suppressed in binaries of
shorter periods. (e) Obtaining data for multiple forbidden lines is important
since different lines probe different characteristic radial scales. Our models
are discussed in relation to ISO data for WR 147 and gamma Vel (WR11). The
lines for WR 147 are probably not accurate enough to draw firm conclusions. For
gamma Vel, individual line morphologies are broadly reproducible but not
simultaneously so for the claimed wind and orbital parameters. Overall, the
effort demonstrates how lines that are sensitive to the large-scale wind can
help to deduce binary system properties and provide new tests of numerical
simulations.Comment: to appear in MNRA
The last gasps of VY CMa: Aperture synthesis and adaptive optics imagery
We present new observations of the red supergiant VY CMa at 1.25 micron, 1.65
micron, 2.26 micron, 3.08 micron and 4.8 micron. Two complementary
observational techniques were utilized: non-redundant aperture masking on the
10-m Keck-I telescope yielding images of the innermost regions at unprecedented
resolution, and adaptive optics imaging on the ESO 3.6-m telescope at La Silla
attaining extremely high (~10^5) peak-to-noise dynamic range over a wide field.
For the first time the inner dust shell has been resolved in the near-infrared
to reveal a one-sided extension of circumstellar emission within 0.1" (~15
R_star) of the star. The line-of-sight optical depths of the circumstellar dust
shell at 1.65 micron, 2.26 micron, and 3.08 micron have been estimated to be
1.86 +/- 0.42, 0.85 +/- 0.20, and 0.44 +/- 0.11. These new results allow the
bolometric luminosity of VY~CMa to be estimated independent of the dust shell
geometry, yielding L_star ~ 2x10^5 L_sun. A variety of dust condensations,
including a large scattering plume and a bow-shaped dust feature, were observed
in the faint, extended nebula up to 4" from the central source. While the
origin of the nebulous plume remains uncertain, a geometrical model is
developed assuming the plume is produced by radially-driven dust grains forming
at a rotating flow insertion point with a rotational period between 1200-4200
years, which is perhaps the stellar rotational period or the orbital period of
an unseen companion.Comment: 25 pages total with 1 table and 5 figures. Accepted by Astrophysical
Journal (to appear in February 1999
A dusty torus around the luminous young star LkHa 101
A star forms when a cloud of dust and gas collapses. It is generally believed
that this collapse first produces a flattened rotating disk, through which
matter is fed onto the embryonic star at the center of the disk. When the
temperature and density at the center of the star pass a critical threshold,
thermonuclear fusion begins. The remaining disk, which can still contain up to
0.3 times the mass of the star, is then sculpted and eventually dissipated by
the radiation and wind from the newborn star. Unfortunately this picture of the
structure and evolution of the disk remains speculative because of the lack of
morphological data of sufficient resolution and uncertainties regarding the
underlying physical processes. Here we present resolved images of a young star,
LkHa 101 in which the structure of the inner accretion disk is resolved. We
find that the disk is almost face-on, with a central gap (or cavity) and a hot
inner edge. The cavity is bigger than previous theoretical predictions, and we
infer that the position of the inner edge is probably determined by sublimation
of dust grains by direct stellar radiation, rather than by disk reprocessing or
the viscous heating processes as usually assumed.Comment: 7 pages, 1 figure. Appears in Nature, 22 Feb, 2001 (Vol 409
Mid-infrared interferometry of the massive young stellar object NGC3603 - IRS 9A
We present observations and models for one of these MYSO candidates, NGC3603
IRS 9A. Our goal is to investigate with infrared interferometry the structure
of IRS 9A on scales as small as 200AU, exploiting the fact that a cluster of O
and B stars has blown away much of the obscuring foreground dust and gas.
Observations in the N-band were carried out with the MIDI beam combiner
attached to the VLTI. Additional interferometric observations which probe the
structure of IRS 9A on larger scales were performed with an aperture mask
installed in the T-ReCS instrument of Gemini South. The spectral energy
distribution (SED) is constrained by the MIDI N-band spectrum and by data from
the Spitzer Space Telescope. Our efforts to model the structure and SED of IRS
9A range from simple geometrical models of the brightness distribution to one-
and two-dimensional radiative transfer computations. The target is resolved by
T-ReCS, with an equivalent (elliptical) Gaussian width of 330mas by 280mas
(2300 AU by 2000 AU). Despite this fact, a warm compact unresolved component
was detected by MIDI which is possibly associated with the inner regions of a
flattened dust distribution. Based on our interferometric data, no sign of
multiplicity was found on scales between about 200AU and 700AU projected
separation. A geometric model consisting of a warm (1000 K) ring (400 AU
diameter) and a cool (140 K) large envelope provides a good fit to the data. No
single model fitting all visibility and photometric data could be found, with
disk models performing better than spherical models. While the data are clearly
inconsistent with a spherical dust distribution they are insufficient to prove
the existence of a disk but rather hint at a more complex dust distribution.Comment: 8 pages, 11 figures. Accepted for publication in A&
3D simulations of RS Oph: from accretion to nova blast
RS Ophiuchi is a recurrent nova with a period of about 22 years, consisting
of a wind accreting binary system with a white dwarf (WD) very close to the
Chandrasekhar limit and a red giant star (RG). The system is considered a prime
candidate to evolve into an SNIa. We present a 3D hydrodynamic simulation of
the quiescent accretion and the subsequent explosive phase. The computed
circumstellar mass distribution in the quiescent phase is highly structured
with a mass enhancement in the orbital plane of about a factor of 2 as compared
to the poleward directions. The simulated nova remnant evolves aspherically,
propagating faster toward the poles. The shock velocities derived from the
simulations are in agreement with those derived from observations. For v_RG =
20 km/s and for nearly isothermal flows, we derive a mass transfer rate to the
WD of 10% of the mass loss of the RG. For an RG mass loss of 10^{-7} solar
masses per year, we found the orbit of the system to decay by 3% per million
years. With the derived mass transfer rate, multi-cycle nova models provide a
qualitatively correct recurrence time, amplitude, and fastness of the nova. Our
simulations provide, along with the observations and nova models, the third
ingredient for a deeper understanding of the recurrent novae of the RS Oph
type. In combination with recent multi-cycle nova models, our results suggests
that the WD in RS Oph will increase in mass. Several speculative outcomes then
seem plausible. The WD may reach the Chandrasekhar limit and explode as an SN
Ia. Alternatively, the mass loss of the RG could result in a smaller Roche
volume, a common envelope phase, and a narrow WD+WD system. Angular momentum
loss due to graviational wave emission could trigger the merger of the two WDs
and - perhaps - an SN Ia via the double degenerate scenario.Comment: Accepted by Astronomy & Astrophysics Letters, 4 pages, 5 figures;
Version with high resolution figures and movie can be found at
http://www.astro.phys.ethz.ch/staff/folini/private/research/rsoph/rsoph.htm
Probing the close environment of young stellar objects with interferometry
The study of Young Stellar Objects (YSOs) is one of the most exciting topics
that can be undertaken by long baseline optical interferometry. The magnitudes
of these objects are at the edge of capabilities of current optical
interferometers, limiting the studies to a few dozen, but are well within the
capability of coming large aperture interferometers like the VLT
Interferometer, the Keck Interferometer, the Large Binocular Telescope or
'OHANA. The milli-arcsecond spatial resolution reached by interferometry probes
the very close environment of young stars, down to a tenth of an astronomical
unit. In this paper, I review the different aspects of star formation that can
be tackled by interferometry: circumstellar disks, multiplicity, jets. I
present recent observations performed with operational infrared
interferometers, IOTA, PTI and ISI, and I show why in the next future one will
extend these studies with large aperture interferometers.Comment: Review to be published in JENAM'2002 proceedings "The Very Large
Telescope Interferometer Challenges for the future
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