204 research outputs found
Optical photometry and spectroscopy of the 1987A-like supernova 2009mw
We present optical photometric and spectroscopic observations of the
1987A-like supernova (SN) 2009mw. Our and photometry covers
167 days of evolution, including the rise to the light curve maximum, and ends
just after the beginning of the linear tail phase. We compare the observational
properties of SN 2009mw with those of other SNe belonging to the same subgroup,
and find that it shows similarities to several objects. The physical parameters
of the progenitor and the SN are estimated via hydrodynamical modelling,
yielding an explosion energy of foe, a pre-SN mass of , a progenitor radius as and a Ni mass
as . These values indicate that the progenitor of SN
2009mw was a blue supergiant star, similar to the progenitor of SN 1987A. We
examine the host environment of SN 2009mw and find that it emerged from a
population with slightly sub-solar metallicty.Comment: 11 pages, 12 figures, accepted for publication in MNRA
Orbital and physical parameters of eclipsing binaries from the ASAS catalogue -- III. Two new low-mass systems with rapidly evolving spots
We present the results of our spectroscopic and photometric analysis of two
newly discovered low-mass detached eclipsing binaries found in the All-Sky
Automated Survey (ASAS) catalogue: ASAS J093814-0104.4 and ASAS J212954-5620.1.
Using the GIRAFFE instrument on the 1.9-m Radcliffe telescope at SAAO and the
UCLES spectrograph on the 3.9-m Anglo-Australian Telescope, we obtained
high-resolution spectra of both objects and derived their radial velocities
(RVs) at various orbital phases. The RVs of both objects were measured with the
TODCOR technique using synthetic template spectra as references. We also
obtained V and I band photometry using the 1.0-m Elizabeth telescope at SAAO
and the 0.4-m PROMPT instruments located at the CTIO. The orbital and physical
parameters of the systems were derived with PHOEBE and JKTEBOP codes. We
compared our results with several sets of widely-used isochrones. Our
multi-epoch photometric observations demonstrate that both objects show
significant out-of-eclipse modulations, which vary in time. We believe that
this effect is caused by stellar spots, which evolve on time scales of tens of
days. For this reason, we constructed our models on the basis of photometric
observations spanning short time scales (less than a month). Our modeling
indicates that (1) ASAS-09 is a main sequence active system with nearly-twin
components with masses of M1 = 0.771(33) Msun, M2 = 0.768(21) Msun and radii of
R1 = 0.772(12) Rsun and R2 = 0.769(13) Rsun. (2) ASAS-21 is a main sequence
active binary with component masses of M1 = 0.833(17) Msun, M2 = 0.703(13) Msun
and radii of R1 = 0.845(12) Rsun and R2 = 0.718(17) Rsun. Both systems confirm
the characteristic of active low-mass stars, for which the observed radii are
larger and the temperatures lower than predicted by evolutionary models. Other
parameters agree within errors with the models of main sequence stars.Comment: 15 pages, 7 figures, 7 tables, to appear in A&
Confirmation of the Luminous Blue Variable status of MWC 930
We present spectroscopic and photometric observations of the emission-line
star MWC 930 (V446 Sct) during its long-term optical brightening in 2006--2013.
Based on our earlier data we suggested that the object has features found in
Luminous Blue Variables (LBV), such as a high luminosity (~3 10^5 Lsun, a low
wind terminal velocity (~ 140 km/s), and a tendency to show strong brightness
variations (~1 mag over 20 years). For the last ~7 years it has been exhibiting
a continuous optical and near-IR brightening along with a change of the
emission-line spectrum appearance and cooling of the star's photosphere. We
present the object's --band light curve, analyze the spectral variations,
and compare the observed properties with those of other recognized Galactic
LBVs, such as AG Car and HR Car. Overall we conclude the MWC 930 is a bona fide
Galactic LBV that is currently in the middle of an S Dor cycle.Comment: 12 pages, 7 figure
Signatures of an eruptive phase before the explosion of the peculiar core-collapse SN 2013gc
We present photometric and spectroscopic analysis of the peculiar
core-collapse SN 2013gc, spanning seven years of observations. The light curve
shows an early maximum followed by a fast decline and a phase of almost
constant luminosity. At +200 days from maximum, a brightening of 1 mag is
observed in all bands, followed by a steep linear luminosity decline after +300
d. In archival images taken between 1.5 and 2.5 years before the explosion, a
weak source is visible at the supernova location, with mag20. The
early supernova spectra show Balmer lines, with a narrow (560 km
s) P-Cygni absorption superimposed on a broad (3400 km s)
component, typical of type IIn events. Through a comparison of colour curves,
absolute light curves and spectra of SN 2013gc with a sample of supernovae IIn,
we conclude that SN 2013gc is a member of the so-called type IId subgroup. The
complex profile of the H line suggests a composite circumstellar medium
geometry, with a combination of lower velocity, spherically symmetric gas and a
more rapidly expanding bilobed feature. This circumstellar medium distribution
has been likely formed through major mass-loss events, that we directly
observed from 3 years before the explosion. The modest luminosity
( near maximum) of SN 2013gc at all phases, the very small amount
of ejected Ni (of the order of M), the major
pre-supernova stellar activity and the lack of prominent [O I] lines in
late-time spectra support a fall-back core-collapse scenario for the massive
progenitor of SN~2013gc.Comment: 20 pages, 11 figures, 8 tables, accepted by MNRA
Fortnightly Fluctuations in the O-C Diagram of CS 1246
Dominated by a single, large-amplitude pulsation mode, the rapidly-pulsating
hot subdwarf B star CS 1246 is a prime candidate for a long-term O-C diagram
study. We collected nearly 400 hours of photometry with the PROMPT telescopes
over a time span of 14 months to begin looking for secular variations in the
pulse timings. Interestingly, the O-C diagram is dominated by a strong
sinusoidal pattern with a period of 14.1 days and an amplitude of 10.7
light-seconds. Underneath this sine wave is a secular trend implying a decrease
in the 371.7-s pulsational period of Pdot = -1.9 x 10^-11, which we attribute
to the evolution of the star through the H-R diagram. The sinusoidal variation
could be produced by the presence of a low-mass companion, with m sin i ~ 0.12
Msun, orbiting the subdwarf B star at a distance of 20 Rsun. An analysis of the
combined light curve reveals the presence of a low-amplitude first harmonic to
the main pulsation mode.Comment: Accepted for publication in MNRAS. 11 pages, 8 figures, 5 table
Optical and X-Ray Observations of GRB 060526: A Complex Afterglow Consistent with An Achromatic Jet Break
We obtained 98 R-band and 18 B, r', i' images of the optical afterglow of GRB
060526 (z=3.21) with the MDM 1.3m, 2.4m, and the PROMPT telescopes in Cerro
Tololo over the 5 nights following the burst trigger. Combining these data with
other optical observations reported in GCN and the Swift-XRT observations, we
compare the optical and X-ray afterglow light curves of GRB 060526. Both the
optical and X-ray afterglow light curves show rich features, such as flares and
breaks. The densely sampled optical observations provide very good coverage at
T>1.e4 sec. We observed a break at 2.4e5 sec in the optical afterglow light
curve. Compared with the X-ray afterglow light curve, the break is consistent
with an achromatic break supporting the beaming models of GRBs. However, the
pre-break and post-break temporal decay slopes are difficult to explain in
simple afterglow models. We estimated a jet angle of \theta_j ~ 7 degrees and a
prompt emission size of R_{prompt} ~ 2e14 cm. In addition, we detected several
optical flares with amplitudes of \Delta m ~ 0.2, 0.6, and 0.2 mag. The X-ray
afterglows detected by Swift have shown complicated decay patterns. Recently,
many well-sampled optical afterglows also show decays with flares and multiple
breaks. GRB 060526 provides an additional case of such a complex, well observed
optical afterglow. The accumulated well-sampled afterglows indicate that most
of the optical afterglows are complex.Comment: Accepted by ApJ, 21 pages, 4 figure
The fading of Cassiopeia A, and improved models for the absolute spectrum of primary radio calibration sources
Based on five years of observations with the 40-foot telescope at Green Bank
Observatory (GBO), Reichart & Stephens (2000) found that the radio source
Cassiopeia A had either faded more slowly between the mid-1970s and late 1990s
than Baars et al. (1977) had found it to be fading between the late 1940s and
mid-1970s, or that it had rebrightened and then resumed fading sometime between
the mid-1970s and mid-1990s, in L band (1.4 GHz). Here, we present 15
additional years of observations of Cas A and Cyg A with the 40-foot in L band,
and three and a half additional years of observations of Cas A, Cyg A, Tau A,
and Vir A with GBO's recently refurbished 20-meter telescope in L and X (9 GHz)
bands. We also present a more sophisticated analysis of the 40-foot data, and a
reanalysis of the Baars et al. (1977) data, which reveals small, but
non-negligible differences. We find that overall, between the late 1950s and
late 2010s, Cas A faded at an average rate of %/yr in L band,
consistent with Reichart & Stephens (2000). However, we also find, at the
6.3 credible level, that it did not fade at a constant rate. Rather,
Cas A faded at a faster rate through at least the late 1960s, rebrightened (or
at least faded at a much slower rate), and then resumed fading at a similarly
fast rate by, at most, the late 1990s. Given these differences from the
original Baars et al. (1977) analysis, and given the importance of their fitted
spectral and temporal models for flux-density calibration in radio astronomy,
we update and improve on these models for all four of these radio sources. In
doing so, we additionally find that Tau A is fading at a rate of
%/yr in L band.Comment: 17 pages, 12 figures, accepted to MNRA
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