224 research outputs found
Two New LBV Candidates in the M33 Galaxy
We present two new luminous blue variable (LBV) candidate stars discovered in
the M33 galaxy. We identified these stars (Valeev et al. 2010) as massive star
candidates at the final stages of evolution, presumably with a notable
interstellar extinction. The candidates were selected from the Massey et al.
(2006) catalog based on the following criteria: emission in Halpha, V<18.5 and
0.35<(B-V)<1.2. The spectra of both stars reveal a broad and strong Halpha
emission with extended wings (770 and 1000 km/s). Based on the spectra we
estimated the main parameters of the stars. Object N45901 has a bolometric
luminosity log(L/Lsun)=6.0-6.2 with the value of interstellar extinction
Av=2.3+-0.1. The temperature of the star's photosphere is estimated as
Tstar~13000-15000K its probable mass on the Zero Age Main Sequence is
M~60-80Msun. The infrared excess in N45901 corresponds to the emission of warm
dust with the temperature Twarm~1000K, and amounts to 0.1% of the bolometric
luminosity. A comparison of stellar magnitude estimates from different catalogs
points to the probable variability of the object N45901. Bolometric luminosity
of the second object, N125093, is log(L/Lsun)=6.3-6.6, the value of
interstellar extinction is Av=2.75+-0.15. We estimate its photosphere's
temperature as Tstar~13000-16000K, the initial mass as M~90-120Msun. The
infrared excess in N125093 amounts to 5-6% of the bolometric luminosity. Its
spectral energy distribution reveals two thermal components with the
temperatures Twarm~1000K and Tcold~480K. The [CaII] lines (7291A and 7323A),
observed in LBV-like stars VarA and N93351 in M33, are also present in the
spectrum of N125093. These lines indicate relatively recent gas eruptions and
dust activity linked with them. High bolometric luminosity of these stars and
broad Halpha emissions allow classifying the studied objects as LBV candidates.Comment: 14 pages, 4 figure
iPTF16fnl: a faint and fast tidal disruption event in an E+A galaxy
We present ground-based and \textit{Swift} observations of iPTF16fnl, a
likely tidal disruption event (TDE) discovered by the intermediate Palomar
Transient Factory (iPTF) survey at 66.6 Mpc. The lightcurve of the object
peaked at absolute mag. The maximum bolometric luminosity (from
optical and UV) was erg/s, an
order of magnitude fainter than any other optical TDE discovered so far. The
luminosity in the first 60 days is consistent with an exponential decay, with
, where =~57631.0 (MJD) and
days. The X-ray shows a marginal detection at erg/s (\textit{Swift} X-ray Telescope). No radio counterpart was
detected down to 3, providing upper limits for monochromatic radio
luminosity of erg/s and erg/s (VLA, 6.1 and 22 GHz). The blackbody temperature, obtained from
combined \textit{Swift} UV and optical photometry, shows a constant value of
19,000 K. The transient spectrum at peak is characterized by broad He II and
H emission lines, with an FWHM of about 14,000 km/s and 10,000 km/s
respectively. He I lines are also detected at 5875 and 6678.
The spectrum of the host is dominated by strong Balmer absorption lines, which
are consistent with a post-starburst (E+A) galaxy with an age of 650 Myr
and solar metallicity. The characteristics of iPTF16fnl make it an outlier on
both luminosity and decay timescales, as compared to other optically selected
TDEs. The discovery of such a faint optical event suggests a higher rate of
tidal disruptions, as low luminosity events may have gone unnoticed in previous
searches.Comment: 14 pages, 11 figures, accepted for publication in Ap
The afterglow and kilonova of the short GRB 160821B
GRB 160821B is a short duration gamma-ray burst (GRB) detected and localized
by the Neil Gehrels Swift Observatory in the outskirts of a spiral galaxy at
z=0.1613, at a projected physical offset of 16 kpc from the galaxy's center. We
present X-ray, optical/nIR and radio observations of its counterpart and model
them with two distinct components of emission: a standard afterglow, arising
from the interaction of the relativistic jet with the surrounding medium, and a
kilonova, powered by the radioactive decay of the sub-relativistic ejecta.
Broadband modeling of the afterglow data reveals a weak reverse shock
propagating backward into the jet, and a likely jet-break at 3.5 d. This is
consistent with a structured jet seen slightly off-axis while expanding into a
low-density medium. Analysis of the kilonova properties suggests a rapid
evolution toward red colors, similar to AT2017gfo, and a low nIR luminosity,
possibly due to the presence of a long-lived neutron star. The global
properties of the environment, the inferred low mass (M_ej < 0.006 Msun) and
velocities (v > 0.05 c) of lanthanide-rich ejecta are consistent with a binary
neutron star merger progenitor.Comment: 14 pages, 6 figures, MNRAS, in press. Moderate revision, added Figure
5 and X-ray data to Table
Doppler confirmation of TESS planet candidate TOI1408.01: grazing transit and likely eccentric orbit
We report an independent Doppler confirmation of the TESS planet candidate
orbiting an F-type main sequence star TOI-1408 located 140 pc away. We present
a set of radial velocities obtained with a high-resolution fiber-optic
spectrograph FFOREST mounted at the SAO RAS 6-m telescope (BTA-6). Our
self-consistent analysis of these Doppler data and TESS photometry suggests a
grazing transit such that the planet obscures its host star by only a portion
of the visible disc. Because of this degeneracy, the radius of TOI-1408.01
appears ill-determined with lower limit about 1 R,
significantly larger than in the current TESS solution. We also derive the
planet mass of ~ and the orbital period
days, thus making this object a typical hot Jupiter, but with a significant
orbital eccentricity of . Our solution may suggest the planet is
likely to experience a high tidal eccentricity migration at the stage of
intense orbital rounding, or may indicate possible presence of other unseen
companions in the system, yet to be detected.Comment: 5 pages, 3 figure
Spectroscopic observations of the exoplanet WASP-32b transit
© 2017, Pleiades Publishing, Ltd.We present first results of spectroscopic observations of transiting exoplanets in the Special Astrophysical Observatory of the Russian Academy of Sciences with the Main Stellar Spectrograph of the 6-m BTA telescope. For the exoplanetWASP-32b, we detected a significant variation of intensity and equivalent width in the Hα spectral line of the parent star at the time of a transit. The equivalent width of the line during transit is by 8–10% larger than outside the planet passage. Residual intensity in the core of the line reveals the following tendency: the line is by 10–15% deeper inside transit than outside it. Observations with the long-slit spectrograph of the Crimean Astrophysical Observatory at the 2.6-m ZTSh telescope also showed a transit event in the Hα line, although, with a smaller amplitude and shape inverted in relation to the data from the 6-m telescope. While in the observations with the BTA the Hα line becomes deeper during the transit, in the ZTSh observations, the residual intensity of the Hα line decreases during the transit. Reducing and analysis of the archive data of WASP-32b observations with the HARPS spectrograph also confirm the Hα line modulation at the time of the transit. The observed data give evidence of the envelope in WASP-32b filling the Roche lobe and a comet-like tail of changing geometry and orientation relative to the observer. These changes determine different depths and shapes of the Hα spectral line at the time of transits
Transition from Fireball to Poynting-flux-dominated Outflow in Three-Episode GRB 160625B
The ejecta composition is an open question in gamma-ray bursts (GRB) physics.
Some GRBs possess a quasi-thermal spectral component in the time-resolved
spectral analysis, suggesting a hot fireball origin. Others show a featureless
non-thermal spectrum known as the "Band" function, consistent with a
synchrotron radiation origin and suggesting that the jet is
Poynting-flux-dominated at the central engine and likely in the emission region
as well. There are also bursts showing a sub-dominant thermal component and a
dominant synchrotron component suggesting a likely hybrid jet composition. Here
we report an extraordinarily bright GRB 160625B, simultaneously observed in
gamma-rays and optical wavelengths, whose prompt emission consists of three
isolated episodes separated by long quiescent intervals, with the durations of
each "sub-burst" being 0.8 s, 35 s, and 212 s, respectively. Its high
brightness (with isotropic peak luminosity L
erg/s) allows us to conduct detailed time-resolved spectral analysis in each
episode, from precursor to main burst and to extended emission. The spectral
properties of the first two sub-bursts are distinctly different, allowing us to
observe the transition from thermal to non-thermal radiation between
well-separated emission episodes within a single GRB. Such a transition is a
clear indication of the change of jet composition from a fireball to a
Poynting-flux-dominated jet.Comment: Revised version reflecting the referees' comments. 27 pages, 11
figures, 5 tables. The final edited version will appear in Nature Astronom
Correct quantum chemistry in a minimal basis from effective Hamiltonians
We describe how to create ab-initio effective Hamiltonians that qualitatively
describe correct chemistry even when used with a minimal basis. The
Hamiltonians are obtained by folding correlation down from a large parent basis
into a small, or minimal, target basis, using the machinery of canonical
transformations. We demonstrate the quality of these effective Hamiltonians to
correctly capture a wide range of excited states in water, nitrogen, and
ethylene, and to describe ground and excited state bond-breaking in nitrogen
and the chromium dimer, all in small or minimal basis sets
Eta Carinae and the Luminous Blue Variables
We evaluate the place of Eta Carinae amongst the class of luminous blue
variables (LBVs) and show that the LBV phenomenon is not restricted to
extremely luminous objects like Eta Car, but extends luminosities as low as
log(L/Lsun) = 5.4 - corresponding to initial masses ~25 Msun, and final masses
as low as ~10-15 Msun. We present a census of S Doradus variability, and
discuss basic LBV properties, their mass-loss behaviour, and whether at maximum
light they form pseudo-photospheres. We argue that those objects that exhibit
giant Eta Car-type eruptions are most likely related to the more common type of
S Doradus variability. Alternative atmospheric models as well as
sub-photospheric models for the instability are presented, but the true nature
of the LBV phenomenon remains as yet elusive. We end with a discussion on the
evolutionary status of LBVs - highlighting recent indications that some LBVs
may be in a direct pre-supernova state, in contradiction to the standard
paradigm for massive star evolution.Comment: 27 pages, 6 figures, Review Chapter in "Eta Carinae and the supernova
imposters" (eds R. Humphreys and K. Davidson) new version submitted to
Springe
Detection of regular low-amplitude photometric variability of the magnetic dwarf WD0009+501. On the possibility of photometric investigation of exoplanets on the basis of 1-meter class telescopes of the special and crimean astrophysical observatories
© 2015, Pleiades Publishing, Ltd. We present the results of photometric observations of the weak magnetic white dwarf WD0009+501. The observations were carried out for two years with the 1-m telescopes of the Special and Crimean Astrophysical Observatories. As a result of these observations, we detected regular V -band luminosity variations with a period of P ≈ 8 hours. The amplitude of the variability is stable on timescales greater than two years and amounts to 11 ± 1 mmag. The difference in the variability amplitude from observations with different telescopes is 1–3 mmag. The result is interpreted within the concept of a rotation-modulated variability of magnetic properties of the star’s atmosphere. We also discuss a possible variability due to the presence of planetary companions around stars of this type. The results of monitoring were used to explore the capabilities of the telescopes for exoplanet investigation. We studied the dependences between the characteristic times of exposures, magnitudes of the objects, and a threshold level of the expected variability amplitudes for all the telescopes involved in our program. A program of exoplanet monitoring with the mentioned telescopes was drawn up for the next few years based on the results of the study
Application of the PM6 method to modeling the solid state
The applicability of the recently developed PM6 method for modeling various properties of a wide range of organic and inorganic crystalline solids has been investigated. Although the geometries of most systems examined were reproduced with good accuracy, severe errors were found in the predicted structures of a small number of solids. The origin of these errors was investigated, and a strategy for improving the method proposed
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