Abstract
We present low-resolution optical spectra and classifications of 92 blue objects with mid-infrared excesses in the Large Magellanic Cloud. The majority of these objects were selected with ...the criteria of
U
−
B
< 0 and
V
< 17 from the potential young stellar object (YSO) candidates in Gruendl & Chu (GC09), which were identified based on Spitzer Infrared Array Camera and Multiband Imaging Photometer for Spitzer observations in conjunction with optical photometry from the Magellanic Clouds Photometric Survey. Many of the sample objects have ambiguous classifications. We examined the properties of these 92 objects using low-resolution optical spectra obtained with the SOAR 4.1 m Telescope at Cerro Pachon and the Blanco 4 m Telescope at Cerro Tololo Inter-American Observatory, supplemented by available photometric and imaging observations. We estimated the spectral types, temperatures, and luminosities of these objects from the optical to near-IR spectral energy distributions based on the photometric data, and further examined stellar absorption line features in the optical spectra to verify the spectral types. The interstellar/circumstellar environments, assessed from nebular line imaging observations and nebular lines detected in the stellar spectra, further helped constrain the nature of stars. Among these 92 objects, we confirm 42 stars as YSOs, and the remaining 50 objects as protoplanetary nebulae, post-AGB/RGB stars, blue evolved massive stars, stars with dust in vicinity, or uncertain classifications. Our results show that the photometric criteria in GC09 are generally effective in the initial selection of YSO candidates, and the low-resolution spectroscopy combined with environment assessment can be useful to better constrain the classifications and ameliorate most ambiguities.
We have used two methods to search for surviving companions of Type Ia supernova progenitors in three Balmer-dominated supernova remnants in the Large Magellanic Cloud: 0519-69.0, 0505-67.9 (DEM ...L71), and 0548-70.4. In the first method, we use the Hubble Space Telescope photometric measurements of stars to construct color-magnitude diagrams (CMDs) and compare positions of stars in the CMDs with those expected from theoretical post-impact evolution of surviving main-sequence or helium star companions. No obvious candidates of surviving companions are identified in this photometric search. Future models for surviving red giant companions or with different explosion mechanisms are needed for thorough comparisons with these observations in order to make more definitive conclusions. In the second method, we use Multi Unit Spectroscopic Explorer observations of 0519-69.0 and DEM L71 to carry out spectroscopic analyses of stars in order to use large peculiar radial velocities as diagnostics of surviving companions. We find a star in 0519-69.0 and a star in DEM L71 moving at radial velocities of 182 0 km s−1 and 213 0 km s−1, respectively, more than 2.5 from the mean radial velocity of the underlying stellar population, 264 and 270 km s−1, respectively. These stars need higher-quality spectra to investigate their abundances and rotation velocities to determine whether they are indeed surviving companions of the supernova progenitors.
Variable Star Network (VSNET) is a global professional-amateur network of researchers in variable stars and related objects, particularly in transient objects, such as cataclysmic variables, ...black-hole binaries, supernovae, and gamma-ray bursts. The VSNET has been playing a pioneering role in establishing the field of transient object astronomy, by effectively incorporating modern advances in observational astronomy and global electronic networks, as well as collaborative progress in theoretical astronomy and astronomical computing. The VSNET is now one of the best-featured global networks in this field of astronomy. We review the historical progress, design concept, associated technology, and a wealth of scientific achievements powered by VSNET.
We report on the time-resolved CCD photometry of a newly discovered variable star, TSS J022216.4
$+$
412259.9 during an outburst in a 2005 November–December brightening. The obtained light curves ...unambiguously show 0.2–0.3 mag modulations, which we confirmed to be the superhump observed among SU UMa-type dwarf novae. We also performed a period search for data obtained during the outburst plateau phase, and revealed the existence of the two periodicities: 0.054868(98)d for the first two nights and 0.055544(26)d for the following plateau phase. This bi-periodicity is hardly observed in the usual SU UMa-type dwarf novae, but is characteristic of WZ Sge-type stars. We undoubtedly detected a rebrightening in the post-outburst stage, which is typical of short-period SU UMa-type dwarf novae, including WZ Sge-type stars. These observations suggest that TSS J022216.4
$+$
412259.9 may be a new WZ Sge star breaking the shortest superhump period so far of 0.05648 d for V592 Her among this class with a known superhump period.
We present near-infrared spectra for five dwarf novae with short orbital periods. We performed fittings of the spectral energy distributions (SEDs) by assuming a power-law distribution for the ...accretion disk and using template spectra of late-type dwarfs for the secondary star. Two are WZ Sge–or WX Cet-type stars, whose spectra show water absorption features, and one of them shows K
$\;$
I and Na
$\;$
I absorption lines in the
$J$
-band. We find that the secondary contributions to the overall SED are larger in WZ Sge stars than in ER UMa stars. The SED fitting suggests M9 and L1 type dwarf companions for our WZ Sge systems, and an M2 dwarf for our ER UMa star. We discuss the direct relationship between the high temperatures of secondary stars and high mass-transfer rates in ER UMa stars. SDSS J013701.06–091234.9 is the only object showing a spectrum dominated by the secondary component. The spectrum strongly suggests that the evolutionary path of SDSS J013701.06–091234.9 is different from that of ordinary CVs, and this object is a candidate of EI Psc-like systems.
We report on time-resolved CCD photometry of the 2005 March/April superoutburst of a recently confirmed SU UMa-type dwarf nova, 1RXS J053234+624755. The superoutburst was accompanied by a precursor ...in which conspicuous superhumps were visible from the second night of the precursor. This is a rare example where superhumps were observed during a precursor. Concerning an estimated mass ratio of 0.07 for the system, the present observations contrast with the well-known Lubow relation. We also found that the superhump period increased as the superoutburst proceeded, despite the fact that the superoutburst showed a precursor. If the superhump-period change is closely related to the disk radius of the superoutburst, systems having a small mass ratio could display an increasing superhump period, even if the superoutburst contains a precursor.
We report on time-resolved photometry of the superoutburst of an SU UMa-type dwarf nova, SDSS J013701.06
$-$
091234.9, in 2003 December–2004 January. The obtained light curves definitely show ...superhumps with a period of 0.056686(12)d, which is one of the shortest superhump periods among those of SU UMa-type dwarf novae ever observed. Considering quiescent photometric studies, we estimated the fractional superhump excess to be 0.024. Spectroscopic observations by Szkody et al. (2003, AJ, 126, 1499) provided evidence for TiO bands despite the short orbital period, implying that the system has a luminous secondary star. We have drawn a color-color diagram of SU UMa-type dwarf novae in quiescence using 2MASS archives, revealing that the location of this star in the color-color diagram deviates from the general trend. The distance to the system was roughly estimated to be
$300 \pm 80 \,\mathrm{pc}$
, using the empirical period–absolute magnitude relation and based on the proper motion.
We examined the VSNET light curve of the unusual SU UMa-type dwarf nova V503 Cyg, which is known to show a short (89 d) supercycle length and exceptionally small (a few) normal outbursts within a ...supercycle. In 1999–2000, V503 Cyg displayed frequent normal outbursts with typical recurrence times of 7–9 d. The behavior during this period is characteristic to an usual SU UMa-type dwarf nova with a short supercycle length. On the other hand, V503 Cyg showed very infrequent normal outbursts in 2001–2002. Some of the superoutbursts during this period were observed to be shorter than usual. The remarkable alternations of the outbursting states in V503 Cyg support the presence of mechanisms of suppressing normal outbursts and prematurely quenching superoutbursts, which have been proposed to explain some unusual SU UMa-type outbursts. The observed temporal variability of the suppressing/quenching mechanisms in the same object suggests that these mechanisms are not primarily governed by a fixed system parameter, but more reflect the state changes in the accretion disk.
We photometrically observed the VY Scl-type cataclysmic variable KR Aurigae after its final rise from a fading episode in 2000–2001. A time-resolved observation revealed that the light curve is ...dominated by a persistent short-term variation with time-scales of minutes to tens of minutes. On some nights, quasi-periodic variations with periods of 10–15 min were observed. No coherent variation was detected. The power spectral density of the variation has a power-law component (
$f^{-1.63}$
). The temporal properties of the short-term variations in KR Aur present additional support for the possibility that flickering in CVs may be better understood as the result of a self-organized critical state, as in black-hole candidates. The light curve lacks “superhump”-type signals, which are relatively frequently seen in VY Scl-type systems and which are suggested to arise from a tidal instability of the accretion disk induced by changing mass-transfer rates. The present observation suggests a borderline of superhump excitation in VY Scl-type stars between mass ratios of
$q = 0.43$
(MV Lyr) and
$q = 0.60$
(KR Aur).
We report on optical observations during the first hour of the gamma-ray burst (GRB) afterglow of GRB 021004. Our observations revealed the existence of a short plateau phase, in which the afterglow ...remained at almost constant brightness, before an ordinary rapid-fading phase. This plateau phase lasted for about 2 hours from 0.024 to 0.10 d after the burst, which corresponds to a missing blank of the early afterglow light curve of GRB 990123. We propose that the plateau phase can be interpreted as being the natural evolution of synchrotron emission from the forward shock region of a blast wave. The time when the typical frequency of the synchrotron emission passes through the optical range has been predicted to be about 0.1 d after the burst, which is consistent with the observed light curve. Our scenario hence implies that the observed feature in GRB 021004 is a common nature of GRB afterglows.
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