Yuxin Xin

The Afterglow of GRB 130427A from 1 to 10^(16) GHz

We present multiwavelength observations of the afterglow of GRB 130427A, the brightest (in total fluence) gamma-ray burst (GRB) of the past 29 yr. Optical spectroscopy from Gemini-North reveals the redshift of the GRB to be z = 0.340, indicating that its unprecedented brightness is primarily the result of its relatively close proximity to Earth; the intrinsic luminosities of both the GRB and its afterglow are not extreme in comparison to other bright GRBs. We present a large suite of multiwavelength observations spanning from 300 s to 130 days after the burst and demonstrate that the afterglow shows relatively simple, smooth evolution at all frequencies, with no significant late-time flaring or rebrightening activity. The entire data set from 1 GHz to 10 GeV can be modeled as synchrotron emission from a combination of reverse and forward shocks in good agreement with the standard afterglow model, providing strong support to the applicability of the underlying theory and clarifying the nature of the GeV emission observed to last for minutes to hours following other very bright GRBs. A tenuous, wind-stratified circumburst density profile is required by the observations, suggesting a massive-star progenitor with a low mass-loss rate, perhaps due to low metallicity. GRBs similar in nature to GRB 130427A, inhabiting low-density media and exhibiting strong reverse shocks, are probably not uncommon but may have been difficult to recognize in the past owing to their relatively faint late-time radio emission; more such events should be found in abundance by the new generation of sensitive radio and millimeter instruments.

Blue Stragglers in Galactic Open Clusters and Integrated Spectral Energy Distributions

Synthetic integrated spectral properties of the old Galactic open clusters are studies in this work, in which 27 Galactic open clusters of ages ≥1 Gyr are selected as the working sample. Based on the photometric observations of these open clusters, a synthetic integrated spectrum has been made for the stellar population of each cluster. The effects of blue straggler (BS) stars on the conventional simple stellar population (SSP) model are analyzed on an individual cluster basis. It is shown that the BSs, whose positions in the color-magnitude diagrams cannot be predicted by the current single-star evolution theory, require significant modifications to the integrated properties of theoretical SSP model. The synthesized integrated spectral energy distributions (ISEDs) of our sample clusters are dramatically different from those of SSPs based on an isochrone only. The BS-corrected ISEDs of stellar populations show systematic enhancements toward shorter wavelengths in the spectra. When measured with broadband colors in unresolvable conditions, the age of a stellar population can be seriously underestimated by the conventional SSP model. Therefore, considering the common existence of BS components in real stellar populations, we should expect considerable alterations of the conventional ISEDs when we apply the technique of evolutionary population synthesis to more complicated stellar systems.

Blue Straggler Stars in Galactic Open Clusters and the Simple Stellar Population Model

The presence of blue straggler stars (BSs) as secure members of Galactic open clusters (OCs) poses a major challenge to the conventional picture of simple stellar population (SSP) models. These are based on the stellar evolution theory of single stars, whereas the major formation mechanisms of BSs are all correlated with stellar interactions. We have illustrated this in a previous study based on a small sample of old (age >= 1 Gyr) Galactic OCs. However, for the purpose of demonstrating the contributions of BSs to the conventional SSP models statistically and systematically, a large database with sufficient coverage of age and metallicity is definitely needed. The working sample now includes 100 Galactic OCs with ages ranging from 0.1 to 10 Gyr. The contributions of BSs to the integrated light of their host clusters are calculated on an individual cluster basis. The general existence of BSs in our star cluster sample dramatically alters the predictions of conventional SSP models in terms of their integrated properties. Neglecting the consequences of nonstandard evolutionary products, such as BSs, in stellar populations, very large uncertainties can be made in analyzing their integrated spectral energy distributions at unresolvable conditions. The current work strongly suggests that when evolutionary population synthesis technique is used to study the properties of unresolved stellar populations in galaxies, the contributions of BSs should be taken into account.

THE BINARY MASS TRANSFER ORIGIN OF THE RED BLUE STRAGGLER SEQUENCE IN M30

Two separated sequences of blue straggler stars (BSSs) have been revealed by Ferraro et al. (2009) in the color-magnitude diagram (CMD) of the Milky Way globular cluster M30. Their presence has been suggested to be related to the two BSS formation channels (namely, collisions and mass-transfer in close binaries) operating within the same stellar system. The blue sequence was indeed found to be well reproduced by collisional BSS models. In contrast, no specific models for mass transfer BSSs were available for an old stellar system like M30. Here we present binary evolution models, including case-B mass transfer and binary merging, specifically calculated for this cluster. We discuss in detail the evolutionary track of a

A Luminous Peculiar Type Ia Supernova SN 2011hr: More Like SN 1991T or SN 2007if?

0.9+0.5 M_\odot

Optical observations of an SN 2002cx-like peculiar supernova SN 2013en in UGC 11369

binary, which spends approximately 4 Gyr in the BSS region of the CMD of a 13 Gyr old cluster. We also run Monte-Carlo simulations to study the distribution of mass transfer BSSs in the CMD and to compare it with the observational data. Our results show that: (1) the color and magnitude distribution of synthetic mass transfer BSSs defines a strip in the CMD that nicely matches the observed red BSS sequence, thus providing strong support to the mass transfer origin for these stars; (2) the CMD distribution of synthetic BSSs never attains the observed location of the blue BSS sequence, thus reinforcing the hypothesis that the latter formed through a different channel (likely collisions); (3) most (

Rapid instrument exchanging system for the Cassegrain focus of the Lijiang 2.4-m Telescope

\sim 60\%

SN 2016coi/ASASSN-16fp: an example of residual helium in a type Ic supernova?

) of the synthetic BSSs are produced by mass-transfer models, while the remaining

Optical Observations of the Young Type Ic Supernova SN 2014L in M99

< 40\%

Variable stars in the field of the old open cluster NGC 188

requires the contribution from merger models.