472 research outputs found
Evolution of V838 Monocerotis during and after the 2002 eruption
By fitting the available photometric data on V838 Mon with standard
supergiant spectra we have derived principal stellar parameters, i.e. effective
temperature, radius and luminosity, and followed the evolution of the object
since its discovery in early January 2002. Our analysis shows that the 2002
outburst of V838 Mon consisted of two major phases: pre-eruption which was
observed in January 2002 and a major outburst, called eruption, which started
in the beginning of February 2002. During pre-eruption the object seemed to be
relaxing after an initial event which had presumably taken place in last days
of December 2001. The eruption phase, which lasted till mid-April 2002,
resulted from a very strong energy burst, which presumably took place in last
days of January at the base of the stellar envelope inflated in pre-eruption.
The burst produced an energy wave, which was observed as a strong luminosity
flash in the beginning of February, followed by a strong mass outflow in form
of two shells, which was observed as an expanding photosphere in later epochs.
In mid-April, when the outflow became optically transparent and most of its
energy radiated away, the object entered the decline phase during which V838
Mon was evolving along the Hayashi track. This we interpret as an evidence that
the main energy source during decline was due to gravitational contraction of
the object envelope inflated in eruption. Late in 2002 a dust formation started
in the expanding shells which gave rise to a strong infrared excess observed in
2003.Comment: 14 pages, 4 figures, 2 tables, accepted in Astronomy & Astrophysic
An analysis of a spectrum of V838 Monocerotis in October 2005
V838 Mon erupted at the beginning of 2002. Among various scenarios proposed
to explain the nature of the outburst, the most promising is a stellar merger
event. The results of spectroscopic observations of the object obtained in
October 2005 with the Keck/HIRES instrument, presented in detail in Paper I,
are analysed and discussed. Our analysis of the molecular bands and the P-Cyg
profiles of atomic lines shows that the object loses matter with a velocity of
up to 215 km/s and a rate of 10^{-6} - 10^{-5} M_sun/yr. In the profiles of
some atomic lines, we have also found evidence of matter infall. A narrow
absorption component, which is particularly strong in some P-Cyg profiles, may
indicate that a jet-like outflow has also been formed. We show that the
observed emission in the [Fe II] lines and an eclipse-like event observed in
November/December 2006 was probably caused by interactions of the expanding
matter, ejected by V838 Mon in 2002, with radiation from the B3V companion. In
particular, the observed profiles of the [Fe II] lines can be easily modelled
in this scenario and allow us to estimate parameters of the system, such as the
position of the B3V companion relative to V838 Mon and the line of sight,
density in the outflowing matter, and mass lost in the 2002 eruption. The
observed appearance of strong H-alpha emission, just before and during the
eclipse-like event, can be interpreted as a result of the accretion of the
outflowing matter onto the B3V companion: the accreted matter, shocked above
the stellar surface, can be a source of extreme-UV and soft X-ray radiation
capable of ionizing and exciting H in the outflow.Comment: 9 pages, 5 figures, accepted in Astronomy & Astrophysic
Strong linear polarization of V4332 Sgr: a dusty disc geometry
The eruption of V4332 Sgr was observed in 1994. During the outburst, the
object became extremely red, so it is considered as belonging to luminous red
transients of the V838 Mon type. It has recently been suggested that the
central object in V4332 Sgr is now hidden in a dusty disc and that the
photospheric spectrum of this object observed in the optical results from
scattering the central star radiation on dust grains in the disc. One expects
significant polarization of the spectrum in this case. We investigate this
prediction. We present and analyse polarimetric observations of V4332 Sgr in
the V and R photometric bands done with the NOT telescope. The optical light of
V4332 Sgr is linearly polarized with a degree of ~26% in the V band and ~11% in
R. Discussion of the observed polarization leads us to conclude that the
photospheric spectrum observed in V4332 Sgr is probably produced by dust
scattering not only in the disc but also in the outflow from the object seen in
the emission features.Comment: accepted in Astronomy & Astrophysics as a research not
Light echo of V838 Monocerotis: properties of the echoing medium
The light echo phenomenon that accompanied the 2002 eruption of V838 Mon
allows one to study the properties of the diffuse dusty matter in the vicinity
of the object. We are aiming at obtaining estimates of the optical thickness of
the circumstellar matter in front of V838 Mon, as well as optical properties of
dust grains in the echoing medium. In particular, we are interested in studying
whether the echoing medium can be responsible for the observed faintness of the
B-type companion of V838 Mon when compared to three B-type stars that are seen
in the vicinty of V838 Mon and are believed to be at the same distance as V838
Mon. We used the V838 Mon light echo images obtained by the Hubble Space
Telescope (HST) in different filters and epochs. From the images we derived the
total brightness of the echo and its surface brightness. The results of the
measurements were compared to model light echoes. The present study allowed us
to estimate the optical thickness of the matter in front of the object and the
mean cosine value of the scattering angle of dust grains in three HST filters.
The optical thickness of the echoing matter is not sufficient to explain the
observed difference in brightness between the B-type companion of V838 Mon and
the other three B-type stars observed in the vicinity of V838 Mon. Implications
of this result are discussed. Our estimate of the mass of the diffuse matter
seen in the light echo shows that the matter cannot have resulted form a past
mass loss activity of V838 Mon. We probably observe remnants of an interstellar
cloud from which V838 Mon and other members of the observed cluster were
formed.Comment: Accepted for publication in Astronomy & Astrophysic
Observations of V838 Mon in the CO rotational lines
We investigate the structure of a field around the position of V838 Mon as
seen in the lowest CO rotational transitions. We also measure and analyse
emission in the same lines at the position of V838 Mon.Observations have
primarily been done in the CO J = 2-1 and J = 3-2 lines using the KOSMA
telescope. A field of 3.4 squared degrees has been mapped in the on-the-fly
mode in these transitions. Longer integration spectra in the on-off mode have
been obtained to study the emission at the position of V838 Mon. Selected
positions in the field have also been observed in the CO J = 1-0 transition
using the Delingha telescope.In the observed field we have identified many
molecular clouds. They can be divided into two groups from the point of view of
their observed radial velocities. One, having V(LSR) in the range 18-32 km/s,
can be identified with the Perseus Galactic arm. The other one, having V(LSR)
between 44-57 km/s, probably belongs to the Norma-Cygnus arm. The radial
velocity of V838 Mon is within the second range but the object does not seem to
be related to any of the observed clouds. We did not find any molecular buble
of a 1 degree dimension around the position of V838 Mon claimed in van Loon et
al. An emission has been detected at the position of the object in the CO J =
2-1 and J = 3-2 transitions. The emission is very narrow (FWHM ~ 1.2 km/s) and
at V(LSR) = 53.3 km/s. Our analysis of the data suggests that the emission is
probably extended.Comment: paper accepted in A&
Violent Stellar Merger Model for Transient Events
We derive the constraints on the mass ratio for a binary system to merge in a
violent process. We find that the secondary to primary stellar mass ratio
should be ~0.003 < (M_2/M_1) < ~0.15. A more massive secondary star will keep
the primary stellar envelope in synchronized rotation with the orbital motion
until merger occurs. This implies a very small relative velocity between the
secondary star and the primary stellar envelope at the moment of merger, and
therefore very weak shock waves, and low flash luminosity. A too low mass
secondary will release small amount of energy, and will expel small amount of
mass, which is unable to form an inflated envelope. It can however produce a
quite luminous but short flash when colliding with a low mass main sequence
star.
Violent and luminous mergers, which we term mergebursts, can be observed as
V838 Monocerotis type events, where a star undergoes a fast brightening lasting
days to months, with a peak luminosity of up to ~10^6 Lo followed by a slow
decline at very low effective temperatures.Comment: Accepted by MNRA
Evolution of the stellar-merger red nova V1309 Scorpii: SED analysis
One very important object for understanding the nature of red novae is V1309
Sco. Its pre-outburst observations showed that, before its red-nova eruption in
2008, it was a contact binary quickly evolving to the merger of the components.
It thus provided us with a direct evidence that the red novae result from
stellar mergers. We analyse the spectral energy distribution (SED) of the
object and its evolution with time. From various optical and infrared surveys
and observing programmes carried out with OGLE, HST, VVV, Gemini South, WISE,
Spitzer, and Herschel we constructed observed SED in 2010 and 2012. Some
limited data are also available for the red-nova progenitor in 2007. We analyse
the data with our model of a dusty envelope surrounding a central star. Dust
was present in the pre-outburst state of V1309 Sco. Its high temperature
(900-1000 K) suggests that this was a freshly formed dust in a presumable
mass-loss from the spiralling-in binary. Shortly after its 2008 eruption,
V1309~Sco became almost completely embedded in dust. The parameters
(temperature, dimensions) of the dusty envelope in 2010 and 2012 evidence that
we then observed matter lost by the object during the 2008 outburst. Its mass
is at least . The object remains quite luminous, although
since its maximum brightness in September 2008, it has faded in luminosity by a
factor of ~50 (in 2012). Far infrared data from Herschel reveal presence of a
cold (~30 K) dust at a distance of a few thousand AU from the object. This
conclusion could be verified by submillimeter interferometric observations.Comment: 12 pages, 7 figures, accepted in Astronomy & Astrophysisc
Optical spectropolarimetry of V4332 Sagittarii
The eruption of V4332 Sgr was observed in 1994. During the outburst, the
object became extremely red, so it has been considered as to belong to red
transients of the V838 Mon type. Optical spectroscopy obtained a few years
after the eruption showed a faint M-type stellar spectrum underlying numerous
molecular and atomic emission features. It has recently been suggested that the
central object in V4332 Sgr is now hidden in a dusty disc and that the
photospheric spectrum of this object observed in the optical results from
scattering of the radiation of the central star on dust grains in the
circumstellar matter. Recent polarimetric photometry has shown that the optical
radiation of the object is indeed strongly polarized. We present and analyse
our spectropolarimetric observations of V4332 Sgr obtained with the VLT in the
optical region. The optical continuum of V4332 Sgr is linearly polarized with a
typical degree of 16.6%. A clear depolarization is observed in the spectral
regions where emission features contribute significantly to the observed flux.
The only prominent exception is the CaI 6573 emission line, which is polarized
in 21%. The results of our spectropolarimetric observations are in accord with
the proposed geometry of the system and the advocated nature of the observed
optical spectrum of V4332 Sgr. The continuum is not seen directly but results
from scattering on dust within the disc and polar outflow, while most of the
gas emission comes from the polar outflow excited by the radiation field of the
central source. Additionally, the observed polarization patterns suggest a
stratification of the outflow.Comment: accepted for publication in Astronomy & Astrophysic
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