68 research outputs found
A model of an expanding giant that swallowed planets for the eruption of V838 Monocerotis
In early 2002 V838 Monocerotis had an extraordinary outburst whose nature is
still unclear. The optical light curve showed at least three peaks and imaging
revealed a light echo around the object - evidence for a dust shell which was
emitted several thousand years ago and now reflecting light from the eruption.
Spectral analysis suggests that the object was relatively cold throughout the
event, which was characterized by an expansion to extremely large radii. We
show that the three peaks in the light curve have a similar shape and thus it
seems likely that a certain phenomenon was three times repeated. Our suggestion
that the outburst was caused by the expansion of a red giant, followed by the
successive swallowing of three relatively massive planets in close orbits,
supplies a simple explanation to all observed peculiarities of this intriguing
object.Comment: 5 pages, 1 LaTex file, 2 .eps figures, accepted for publication in
MNRA
The properties of V838 Mon in 2002 November
We present the results of modelling the 0.45--1 micron spectral energy
distribution of V838 Mon for 2002 November. Synthetic spectra were calculated
using the NextGen model atmospheres of Hauschildt et al. (1999), which
incorporate line lists for H2O, TiO, CrH, FeH, CO, and MgH, as well as the VALD
atomic line list. Fits to the observed spectra show that, in 2002 November, the
effective temperature of V838 Mon was approximately 2000 +/-100 K. Our
theoretical spectra show a comparatively weak dependence on log g. Preliminary
analysis of the hot star observed together with V838 Mon shows it to be a
normal B3V dwarf.Comment: 7 pages, 4 figs, accepted by A&
How peculiar is the ‘peculiar variable’ DZ Crucis (Nova Cru 2003)?
The variable star DZ Cru was thought to be a nova when it was discovered in eruption in 2003 August. This explanation was later challenged, however, when the first spectra of the object were reported. We present near-infrared spectroscopy of DZ Cru obtained at the New Technology Telescope on three occasions, starting ∼1.5 yr after outburst, with the aim of establishing the nature of the object. The spectra display H i, O i, [N i] emission lines, together with He i P Cygni lines superposed on a dust continuum. These observations suggest the ‘peculiar variable in Crux’ is a classical nova
TW Hya: Spectral Variability, X-Rays, and Accretion Diagnostics
The nearest accreting T Tauri star, TW Hya was observed with spectroscopic
and photometric measurements simultaneous with a long se gmented exposure using
the CHANDRA satellite. Contemporaneous optical photometry from WASP-S indicates
a 4.74 day period was present during this time. Absence of a similar
periodicity in the H-alpha flux and the total X-ray flux points to a different
source of photometric variations. The H-alpha emission line appears
intrinsically broad and symmetric, and both the profile and its variability
suggest an origin in the post-shock cooling region. An accretion event,
signaled by soft X-rays, is traced spectroscopically for the first time through
the optical emission line profiles. After the accretion event, downflowing
turbulent material observed in the H-alpha and H-beta lines is followed by He I
(5876A) broadening. Optical veiling increases with a delay of about 2 hours
after the X-ray accretion event. The response of the stellar coronal emission
to an increase in the veiling follows about 2.4 hours later, giving direct
evidence that the stellar corona is heated in part by accretion. Subsequently,
the stellar wind becomes re-established. We suggest a model that incorporates
this sequential series of events: an accretion shock, a cooling downflow in a
supersonically turbulent region, followed by photospheric and later, coronal
heating. This model naturally explains the presence of broad optical and
ultraviolet lines, and affects the mass accretion rates determined from
emission line profiles.Comment: 61 pages; 22 figures; to appear in The Astrophysical Journa
VLTI observations of the dust geometry around R Coronae Borealis stars
We are investigating the formation and evolution of dust around the
hydrogen-deficient supergiants known as R Coronae Borealis (RCB) stars. We aim
to determine the connection between the probable merger past of these stars and
their current dust-production activities. We carried out high-angular
resolution interferometric observations of three RCB stars, namely RY Sgr, V
CrA, and V854 Cen with the mid-IR interferometer, MIDI on the VLTI, using two
telescope pairs. The baselines ranged from 30 to 60 m, allowing us to probe the
dusty environment at very small spatial scales (~ 50 mas or 400 stellar radii).
The observations of the RCB star dust environments were interpreted using both
geometrical models and one-dimensional radiative transfer codes. From our
analysis we find that asymmetric circumstellar material is apparent in RY Sgr,
may also exist in V CrA, and is possible for V854 Cen. Overall, we find that
our observations are consistent with dust forming in clumps ejected randomly
around the RCB star so that over time they create a spherically symmetric
distribution of dust. However, we conclude that the determination of whether
there is a preferred plane of dust ejection must wait until a time series of
observations are obtained.Comment: Accepted for publication in MNRAS; 14 pages, 10 figures, 6 table
Polarized QPOs from the INTEGRAL polar IGRJ14536-5522 (=Swift J1453.4-5524)
We report optical spectroscopy and high speed photometry and polarimetry of
the INTEGRAL source IGRJ14536-5522 (=Swift J1453.4-5524). The photometry,
polarimetry and spectroscopy are modulated on an orbital period of 3.1564(1)
hours. Orbital circularly polarized modulations are seen from 0 to -18 per
cent, unambiguously identifying IGRJ14536-5522 as a polar.
Some of the high speed photometric data show modulations that are consistent
with quasi-periodic oscillations (QPOs) on the order of 5-6 minutes.
Furthermore, for the first time, we detect the (5-6) minute QPOs in the
circular polarimetry. We discuss the possible origins of these QPOs. We also
include details of HIPPO, a new high-speed photo-polarimeter used for some of
our observations.Comment: Accepted for publication by MNRAS. The paper contains 7 figures and 1
tabl
Observation of light echoes around very young stars
The goal of the paper is to present new results on light echoes from young
stellar objects. Broad band CCD images were obtained over three months at
one-to-two week intervals for the field of NGC 6726, using the large
field-of-view remotely-operated telescope on top of Cerro Burek. We detected
scattered light echoes around two young, low-amplitude, irregular variable
stars. Observations revealed not just one, but multiple light echoes from
brightness pulses of the T Tauri star S CrA and the Herbig Ae/Be star R CrA.
Analysis of S CrA's recurring echoes suggests that the star is located 138 +/-
16 pc from Earth, making these the closest echoes ever detected. The
environment that scatters the stellar light from S CrA is compatible with an
incomplete dust shell or an inclined torus some 10,000 AU in radius and
containing M_{\sun} of dust. The cause of such
concentration at 10,000AU from the star is unknown. It could be the
remnant of the envelope from which the star formed, but the distance of the
cloud is remarkably similar to the nominal distance of the Oort cloud to the
Sun, leading us to also speculate that the dust (or ice) seen around S CrA
might have the same origin as the Solar System Oort cloud.Comment: A&A, in press Received: 16 March 2010 / Accepted: 01 June 201
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