J.-Y. Hu

A High Intrinsic Peculiarity Rate among Type Ia Supernovae

We have compiled a sample of 45 Type Ia supernovae (SNe Ia) discovered by the Lick Observatory Supernova Search (LOSS) and the Beijing Astronomical Observatory Supernova Survey (BAOSS), and determined the rate of spectroscopically peculiar SNe Ia (i.e., SN 1986G-like, SN 1991bg-like, and SN 1991T-like objects) and the luminosity function of SNe Ia. Because of the nature of the two surveys (distance-limited with small baselines and deep limiting magnitudes), nearly all SNe Ia have been discovered in the sample galaxies of LOSS and BAOSS; thus, the observed peculiarity rate and luminosity function of SNe Ia are intrinsic. We find that 36% ± 9% of nearby SNe Ia are peculiar; specifically, the luminosity function of SNe Ia consists of 20% SN 1991T-like, 64% normal, and 16% SN 1991bg-like objects. We have compared our results to those found by earlier studies, and to those found at high redshift. The apparent dearth of SN 1991T-like objects at high redshift may be due to extinction, and especially to the difficulty of recognizing them from spectra obtained past maximum brightness or from spectra with low signal-to-noise ratios. Implications of the high peculiarity rate for the progenitor systems of SNe Ia are also briefly discussed.

The Rise Time of Nearby Type Ia Supernovae

The work at the University of California, Berkeley, was supported by the Miller Institute for Basic Research in Science, by NSF grant AST 94-17213, and by grant GO-7505 from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

Triggered Star Formation in the Orion Bright-rimmed Clouds

We have developed an empirical and effective set of criteria, based on the Two Micron All Sky Survey (2MASS) colors, to select candidate classical T Tauri stars (CTTSs). This provides a useful tool to study the young stellar population in star-forming regions. Here we present our analysis of the bright-rimmed clouds (BRCs) B35, B30, IC 2118, LDN 1616, LDN 1634, and Ori East to show how massive stars interact with molecular clouds to trigger star formation. Our results support the radiation-driven implosion model, in which the ionization fronts from OB stars compress a nearby cloud until the local density exceeds the critical value, thereby inducing the cloud to collapse to form stars. We find that only BRCs associated with strong IRAS 100 μm emission (a tracer of high density) and Hα emission (a tracer of ionization fronts) show signs of ongoing star formation. Relevant timescales, including the ages of O stars, expanding H II regions, and the ages of CTTSs, are consistent with sequential star formation. We also find that CTTSs are only seen between the OB stars and the BRCs, with those closer to the BRCs being progressively younger. There are no CTTSs leading the ionization fronts, i.e., within the molecular clouds. All of these findings provide strong evidence of triggered star formation and show the major roles massive stars play in sustaining the star-forming activities in the region.

The study of a Type IIb supernova: SN 1996cb

We present spectroscopic and photometric results of SN 1996cb. The supernova was independently discovered in NGC 3510 by M. Aoki, T. Cho, & K. Toyama of Japan and Qiao et al. of Beijing Astronomical Observatory on 1996 December 15 and 18, respectively. The results cover about 6 months following the discovery. The first few spectra showed strong Balmer lines with obvious P Cygni profiles, offering evidence of a Type II supernova. The emergence of He I lines could be inferred in these spectra. That the He I lines became quite prominent in the spectra near optical maximum confirmed that SN 1996cb was definitely a Type IIb supernova, like SN 1987K and SN 1993J. The photometric results showed that the B-I color evolution was very similar to that of SN 1993J. Comparing two color curves, we were able to estimate that the explosion of SN 1996cb occurred on UT 1996 December 12. Although the overall light curves resembled that of SN 1993J, they showed some differences, especially for the B band. SN 1996cb had a broad peak in the light curves, and it declined somewhat slowly and, compared with SN 1993J, exhibited a plateau-like shape between 20 and 50 days after the maximum for the B and V bands. This indicates that there was relatively more hydrogen in the outer envelope of the progenitor of SN 1996cb. The spectral evolution of SN 1996cb displayed further differences from SN 1993J. In the case of SN 1996cb, the Balmer lines showed strong P Cygni profiles at a very early time, resembling the early spectra of SN 1987A, a supernova resulting from a compact blue supergiant star. The dramatic changes of expansion velocities at early times indicated that the photosphere of SN 1996cb receded more quickly than it did in SN 1993J. The He I lines emerged much earlier and evolved more dramatically, causing SN 1996cb to display the features of a Type Ib supernova before maximum. This might be the result of a dramatic recession of the photosphere at an early time; the He I lines and [O I] lines showed conspicuous blueshifts when they emerged, resembling the blueshifts of the [O I] lines that appeared in the late spectra of SN 1993J. This is probably observational evidence of Rayleigh-Taylor instabilities occurring at the interfaces between the H and He and the He and O+C layers, respectively; Hα emission and absorption components, especially the latter, were conspicuous 100 days after the explosion. We also conclude that the outer envelope of SN 1996cb had relatively more hydrogen than was the case for SN 1993J, even though the amount remained much less than is typical of other Type II supernovae. This finding is consistent with the results of the photometry. The [O I] and O lines emerged very late and exhibited weak emission, indicating that the He-rich layer was relatively thick. Combining the analyses of photometric and spectroscopic evolution, we conclude that the progenitor of SN 1996cb, like that of SN 1993J, was a stripped massive star exhibiting some special features: it was probably a more compact star with a thick helium layer and a relatively more massive hydrogen envelope.

Spectrum Analysis of the Type Ib Supernova SN 1999dn: Probable Identifications of C II and Hα

Low-resolution spectra of SN 1999dn at early times are presented and compared with synthetic spectra generated with the parameterized supernova synthetic spectrum code SYNOW. We find that the spectra of SN 1999dn strongly resemble those of SN 1997X and SN 1984L, and hence we classify it as a Type Ib event. Line identifications are established through spectrum synthesis. Strong evidence of both Hα and C II λ6580 is found. We infer that Hα appears first, before the time of maximum brightness, and then is blended with and finally overwhelmed by the C II line after maximum; this favors a thin high-velocity hydrogen skin in this Type Ib supernova.

The PLATO Dome A Site-Testing Observatory : instrumentation and first results

The PLATeau Observatory (PLATO) is an automated self-powered astrophysical observatory that was deployed to Dome A, the highest point on the Antarctic plateau, in 2008 January. PLATO consists of a suite of site-testing instruments designed to quantify the benefits of the Dome A site for astronomy, and science instruments designed to take advantage of the unique observing conditions. Instruments include CSTAR, an array of optical telescopes for transient astronomy; Gattini, an instrument to measure the optical sky brightness and cloud cover statistics; DASLE, an experiment to measure the statistics of the meteorological conditions within the near-surface layer; Pre-HEAT, a submillimeter tipping radiometer measuring the atmospheric transmission and water vapor content and performing spectral line imaging of the Galactic plane; and Snodar, an acoustic radar designed to measure turbulence within the near-surface layer. PLATO has run completely unattended and collected data throughout the winter 2008 season. Here we present a detailed description of the PLATO instrument suite and preliminary results obtained from the first season of operation.

Chinese Small Telescope ARray (CSTAR) for Antarctic Dome A

Chinese first arrived in Antarctic Dome A in Jan. 2005 where is widely predicted to be a better astronomical site than Dome C where have a median seeing of 0.27arcsec above 30m from the ground. This paper introduces the first Chinese Antarctic telescope for Dome A (CSTAR) which is composed of four identical telescopes, with entrance pupil 145 mm, 20 square degree FOV and four different filters g, r, i and open band. CSTAR is mainly used for variable stars detection, measurement of atmosphere extinction, sky background and cloud coverage. Now CSTAR has been successfully deployed on Antarctic Dome A by the 24th Chinese expedition team in Jan. 2008. It has started automatic observation since March 20, 2008 and will continuously observe the south area for the whole winter time. The limited magnitude observed is about 16.5m with 20 seconds exposure time. CSTARSs success is a treasurable experience and we can benefit a lot for our big telescope plans, including our three ongoing 500mm Antarctic Schmidt telescopes (AST3).

The PLATO Antarctic site testing observatory

Over a decade of site testing in Antarctica has shown that both South Pole and Dome C are exceptional sites for astronomy, with certain atmospheric conditions superior to those at existing mid-latitude sites. However, the highest point on the Antarctic plateau, Dome A, is expected to experience colder atmospheric temperatures, lower wind speeds, and a turbulent boundary layer that is confined closer to the ground. The Polar Research Institute of China, who were the first to visit the Dome A site in January 2005, plan to establish a permanently manned station there within the next decade. As part of this process they conducted a second expedition to Dome A, arriving via overland traverse in January 2008. This traverse involved the delivery and installation of the PLATeau Observatory (PLATO). PLATO is an automated self-powered astrophysical site testing observatory, developed by the University of New South Wales. A number of international institutions have contributed site testing instruments measuring turbulence, optical sky background, and sub-millimetre transparency. In addition, a set of science instruments are providing wide-field high time resolution optical photometry and terahertz imaging of the Galaxy. We present here an overview of the PLATO system design and instrumentation suite.

The First Release of the CSTAR Point Source Catalog from Dome A, Antarctica

In 2008 January the twenty-fourth Chinese expedition team successfully deployed the Chinese Small Telescope ARray (CSTAR) to Dome A, the highest point on the Antarctic plateau. CSTAR consists of four 14.5 cm optical telescopes, each with a different filter (g, r, i, and open) and has a 4.5° × 4.5° field of view (FOV). It operates robotically as part of the Plateau Observatory, PLATO, with each telescope taking an image every ~30 s throughout the year whenever it is dark. During 2008, CSTAR 1 performed almost flawlessly, acquiring more than 0.3 million i-band images for a total integration time of 1728 hr during 158 days of observations. For each image taken under good sky conditions, more than 10,000 sources down to ~16th magnitude could be detected. We performed aperture photometry on all the sources in the field to create the catalog described herein. Since CSTAR has a fixed pointing centered on the south celestial pole (decl. = -90°), all the sources within the FOV of CSTAR were monitored continuously for several months. The photometric catalog can be used for studying any variability in these sources, and for the discovery of transient sources such as supernovae, gamma-ray bursts, and minor planets.

Sky Brightness and Transparency in the i-band at Dome A, Antarctica

The i-band observing conditions at Dome A on the Antarctic plateau have been investigated using data acquired during 2008 with the Chinese Small Telescope Array. The sky brightness, variations in atmospheric transparency, cloud cover, and the presence of aurorae are obtained from these images. The median sky brightness of moonless clear nights is 20.5 mag arcsec(-2) in the SDSS i band at the south celestial pole (which includes a contribution of about 0.06 mag from diffuse Galactic light). The median over all Moon phases in the Antarctic winter is about 19.8 mag arcsec(-2). There were no thick clouds in 2008. We model contributions of the Sun and the Moon to the sky background to obtain the relationship between the sky brightness and transparency. Aurorae are identified by comparing the observed sky brightness to the sky brightness expected from this model. About 2% of the images are affected by relatively strong aurorae.