22 research outputs found
The post-outburst photometric behaviour of V838 Mon
The unusual eruptive variable discovered in Monoceros in 2002 January
underwent dramatic photometric and spectroscopic changes in the months prior to
its 2002 June-August conjunction with the Sun. Optical and infrared (IR)
photometry obtained at the South African Astronomical Observatory (SAAO)
between 2002 January and June (JD 2452280-440) is presented here in an analysis
of the star's post-outburst behaviour. The light curve indicated 3 eruptions
took place in 2002 January, February and March. SAAO echelle spectra obtained
in the week prior to the March maximum indicated the ejection of a new shell of
material. JHKL photometry obtained during 2002 April showed the development of
an IR excess due to the formation of a dust shell. The shell appears to be
largely responsible for the rapid fade in the optical flux during 2002
April-May (Delta V > 6 mag within 3 weeks). Blueing of the optical colours
during the decline is likely due either to the revealing of an emission line
region surrounding V838 Mon, or the unveiling of the progenitor or a
spatially-close early-type star.Comment: 7 pages, 7 figures - accepted for MNRA
From Top to Bottom - the Multiwavelength Campaign of V824 Ara (HD 155555)
A great deal of progress has been made in recent years in decomposing the 2-D structure in the atmospheres of late-type stars. Doppler images of many photospheres single stars, T Tauri stars, Algols, RS CV(sub n) binaries to name a few - are regularly published (Strassmeier 1996; Richards and Albright 1996; Rice and Strassmeier 1996; Kuerster et al. 1994). Ultraviolet spectral images of chromospheres appear in the literature (e.g., Walter et al. 1987; Neff et al. 1989) but are less common owing to the difficult nature of obtaining complete phase coverage. Zeeman doppler images of magnetic fields are now feasible (e.g., Donati et al. 1992). Performing Doppler imaging of the same targets over many seasons has also been accomplished (e.g, Vogt et al. 1997). Even when a true image reconstruction is not possible due to poor spectral resolution, we can still infer a great deal about spatial structure if enough phases are observed. However, it is increasingly apparent that to make sense of recent results, many different spectral features spanning a range of formation temperature and density must be observed simultaneously for a coherent picture to emerge. Here we report on one such campaign. In 1996, we observed the southern hemisphere RS CV(sub n) binary V824 Ara (P=1(sup d).68, G5IV+K0V-IV-IV) over one complete stellar rotation with the Hubble Space Telescope and EUVE. In conjunction, radio and optical photometry and spectroscopy were obtained from the ground. Unique to this campaign is the complete phase coverage of a number of activity proxy indicators that cover source temperatures ranging from the photosphere to the corona
V838 Mon: light echo evolution and distance estimate
Following its 2002 February eruption, V838 Mon developed a light echo that
continues to expand and evolve as light from the outburst scatters off
progressively more distant circumstellar and/or interstellar material.
Multi-filter images of the light echo, obtained with the South African
Astronomical Observatory (SAAO) 1.0-m telescope between 2002 May and 2004
December, are analysed and made available electronically. The expansion of the
light echo is measured from the images and the data compared with models for
scattering by a thin sheet and a thin shell of dust. From these model results
we infer that the dust is likely in the form of a thin sheet distant from the
star, suggesting that the material is of interstellar origin, rather than being
from earlier stages in the star's evolution. Although the fit is uncertain, we
derive a stellar distance of ~ 9 kpc and a star-dust distance of ~ 5 pc, in
good agreement with recent results reported from other methods. We also present
JHKL and Cousins UBVRI photometry obtained at the SAAO during the star's
second, third and fourth observing seasons post-outburst. These data show
complex infrared colour behaviour while V838 Mon is slowly brightening in the
optical.Comment: 9 pages, 6 figures - accepted for publication in MNRA
Dust-enshrouded Asymptotic Giant Branch Stars in the Solar Neighbourhood
A study is made of a sample of 58 dust-enshrouded Asymptotic Giant Branch
(AGB) stars (including 2 possible post AGB stars), of which 27 are carbon-rich
and 31 are oxygen-rich. These objects were originally identified by Jura &
Kleinmann as nearby (within about 1 kpc of the sun) AGB stars with high
mass-loss rates, greater than 1E-6 solar masses per year. Ground-based
near-infrared photometry, data obtained by IRAS and kinematic data from the
literature are combined to investigate the properties of these stars. The light
amplitude in the near-infrared is found to be correlated with period, and this
amplitude decreases with increasing wavelength. Statistical tests show that
there is no reason to suspect any difference in the period distributions of the
carbon- and oxygen-rich stars for periods less than 1000 days, and no
carbon-rich star has a period longer than 1000 days. The colours are consistent
with those of cool stars with evolved circumstellar dust-shells. Luminosities
and distances are estimated using a period-luminosity relation. Mass-loss
rates, estimated from the 60 micron fluxes, show a correlation with pulsation
period and is tightly correlated with the K-[12] colour. The kinematics and
scale-height of the sample shows that the sources with periods less than 1000
days must have low mass main-sequence progenitors. It is argued that the three
oxygen-rich stars with periods over 1000 days probably had intermediate mass
main-sequence progenitors with remaining stars having an average progenitor
mass of about 1.3 solar masses. The average lifetime of stars in this phase is
estimated to be about 4.0E4 years, indicating they will undergo at most one
more thermal pulse before leaving the AGB.Comment: 27 pages, 20 figures, accepted for MNRA
The fall and rise of V854 Centauri: long-term ultraviolet spectroscopy of a highly-active R Coronae Borealis star
We examine long-term low-dispersion IUE, SWP and LWP spectroscopy of the R
Coronae Borealis (RCB) star V854 Cen, obtained across the deep 1991, 1992-1993
and 1994 declines. We also report the optical light curve for the star in the
interval 1987-1998, including multi-color photometry obtained during 1989-1998.
Analysis of the UV emission line spectra indicates most lines decay during
the deep declines on characteristic timescales comparable to that reported for
optical features. Fe, Mg and neutral C lines decay on timescales of typically
50-100 d. Other lines, notably ionized C lines, decay on longer timescales (>
200 d) or appear to be unaffected by the declines. The general nature of the UV
emission lines and other UV features during the declines is consistent with the
E1/E2/BL line-region model developed from the behavior of optical spectral
features during declines. However, the detailed line-behavior indicates large
intrinsic variability between decline events inconsistent with the simple
E1/E2/BL model. Limited temporal coverage prevents detailed examination of the
geometry of the emission line region or the obscuring dust. We also report the
first detection of the transition-region line C IV 1550 in the spectrum of an
RCB star.Comment: AJ in press (June), 7 figures, 4 table
Recurrent dust formation by WR 48a on a 30-year timescale
We present infrared photometry of the WC8 Wolf-Rayet system WR 48a observed
with telescopes at ESO, the SAAO and the AAT between 1982 and 2011 which show a
slow decline in dust emission from the previously reported outburst in 1978--79
until about 1997, when significant dust emission was still evident. This was
followed by a slow rise, accelerating to reach and overtake the first (1978)
photometry, demonstrating that the outburst observed in 1978--79 was not an
isolated event, but that they recur at intervals of 32+ years. This suggests
that WR 48a is a long-period dust maker and colliding-wind binary (CWB). The
locus of WR 48a in the (H-L), K colour-magnitude diagram implies that the rate
of dust formation fell between 1979 and about 1997 and then increased steadily
until 2011. Superimposed on the long-term variation are secondary (`mini')
eruptions in (at least) 1990, 1994, 1997, 1999 and 2004, characteristic of
relatively brief episodes of additional dust formation. Spectra show evidence
for an Oe or Be companion to the WC8 star, supporting the suggestion that WR
48a is a binary system and indicating a system luminosity consistent with the
association of WR 48a and the young star clusters Danks 1 and Danks 2. The
range of dust formation suggests that these stars are in an elliptical orbit
having e ~ 0.6. The size of the orbit implied by the minimum period, together
with the WC wind velocity and likely mass-loss rate, implies that the
post-shock WC wind is adiabatic throughout the orbit -- at odds with the
observed dust formation. A similar conflict is observed in the `pinwheel'
dust-maker WR 112.Comment: 15 pages, 8 figures, accepted for publication in MNRA
Multiwavelength Monitoring of the BL Lacertae Object PKS 2155-304 in May 1994. I. The Ground-Based Campaign
Optical, near-infrared, and radio observations of the BL Lac object
PKS2155-304 were obtained simultaneously with a continuous UV/EUV/X-ray
monitoring campaign in 1994 May. Further optical observations were gathered
throughout most of 1994. The radio, millimeter, and near-infrared data show no
strong correlations with the higher energies. The optical light curves exhibit
flickering of 0.2-0.3 mag on timescales of 1-2 days, superimposed on longer
timescale variations. Rapid variations of ~0.01 mag/min, which, if real, are
the fastest seen to date for any BL Lac object. Small (0.2-0.3 mag) increases
in the V and R bands occur simultaneously with a flare seen at higher energies.
All optical wavebands (UBVRI) track each other well over the period of
observation with no detectable delay. For most of the period the average colors
remain relatively constant, although there is a tendency for the colors (in
particular B-V) to vary more when the source fades. In polarized light, PKS
2155-304 showed strong color dependence and the highest optical polarization (U
= 14.3%) ever observed for this source. The polarization variations trace the
flares seen in the ultraviolet flux.Comment: 45 pages, latex file with encapsulated postscript, accepted to the
Astrophysical Journa