Opening the 100-Year Window for Time Domain Astronomy

The large-scale surveys such as PTF, CRTS and Pan-STARRS-1 that have emerged within the past 5 years or so employ digital databases and modern analysis tools to accentuate research into Time Domain Astronomy (TDA). Preparations are underway for LSST which, in another 6 years, will usher in the second decade of modern TDA. By that time the Digital Access to a Sky Century @ Harvard (DASCH) project will have made available to the community the full sky Historical TDA database and digitized images for a century (1890--1990) of coverage. We describe the current DASCH development and some initial results, and outline plans for the "production scanning" phase and data distribution which is to begin in 2012. That will open a 100-year window into temporal astrophysics, revealing rare transients and (especially) astrophysical phenomena that vary on time-scales of a decade. It will also provide context and archival comparisons for the deeper modern surveysComment: 6 pages, 3 figures; invited talk at IAUS 28

Exploring the Long-Term and Extreme Variability of Stars

This thesis presents observational studies of long-term and extreme variability of stars with the Digital Access to a Sky Century@Harvard (DASCH) project. Stellar variations over decades are poorly explored. With the unique 100 years coverage of DASCH, for the first time, we are able to study the variable sky over long timescales in a systematic way. I have developed photometric calibration and variable search algorithms for DASCH. I have discovered exciting new types of long-term variables, which do not match any of the common classes, and studied the physical processes involved. Following a brief introduction on variable stars and DASCH in the first chapter, I describe my work on DASCH pipeline, including photometric development and defect filtering in chapter 2. I present our discovery of a group of peculiar long-term K giant variables with $\sim1$ mag variations over decades in chapter 3. Follow-up observations show that they consist of two subgroups, including a subgroup of RS CVn binaries with strong magnetic activity, and another subgroup of single stars. In both cases, the variation amplitudes and timescales are abnormal, and may be related to either ultra strong star spots, or novel dust formation processes. In chapter 4, I present the discovery of a 5 yr dip around 1900 in the eclipsing binary KU Cyg consisting of a F star and a K giant, which is related to the accretion disk surrounding the F star. It showed a slow fading $(\sim 4 yr)$, which is probably caused by increases in dust extinction in the disk, and a relatively fast brightening $(\sim 1 yr)$, which may be due to the evaporation of dust transported inward through the disk. The extinction excess which caused the fading may arise from an increased mass transfer rate in the system or from dust clump ejections from the K giant, in accordance with K giant “dimming” as discussed above. In chapter 5, I present a 10 yr nova-like outburst in a peculiar symbiotic system. With P = 119 days, it is interestingly located in the period gap region between classical novae and symbiotic novae. The most probable explanation of the outburst is hydrogen shell-burning on the white dwarf (WD) without significant mass loss, which suggests a promising new channel for Type Ia Supernovae (SNe). In chapter 6, I present the DASCH light curves of Kepler planet-candidate host stars. We found no variation for these host stars. In chapter 7, I present my variable search algorithms and the resulting DASCH variable catalog for the Kepler field. The conclusion is presented in chapter 8.Astronom

Improved Photometry for the DASCH Pipeline

The Digital Access to a Sky Century@Harvard (DASCH) project is digitizing the ∼500,000 glass plate images obtained (full sky) by the Harvard College Observatory from 1885 to 1992. Astrometry and photometry for each resolved object are derived with photometric rms values of ∼0.15 mag for the initial photometry analysis pipeline. Here we describe new developments for DASCH photometry, applied to the Kepler field, that have yielded further improvements, including better identification of image blends and plate defects by measuring image profiles and astrometric deviations. A local calibration procedure using nearby stars in a similar magnitude range as the program star (similar to what has been done for visual photometry from the plates) yields additional improvement for a net photometric rms of ∼0.1 mag. We also describe statistical measures of light curves that are now used in the DASCH pipeline processing to identify new variables autonomously. The DASCH photometry methods described here are used in the pipeline processing for the data releases of DASCH data, as well as for a forthcoming paper on the long-term variables discovered by DASCH in the Kepler field

Is the dependence of spectral index on luminosity real in optically selected AGN samples?

We critically examine the dependence of spectral index on luminosity in optically selected AGN samples. An analysis of optically selected high-redshift quasars showed an anti-correlation of $\alpha_{OX}$, the spectral index between the rest-frame 2500 A and 2 keV, with optical luminosity (Miyaji et al. 2006). We examine this relationship by means of Monte Carlo simulations and conclude that a constant spectral index independent of optical luminosity is still consistent with this high-z sample. We further find that that contributions of large dispersions and narrow range of optical luminosity are most important for the apparent, yet artificial, $\alpha_{OX} - l_o$ correlation reported. We also examine another, but more complete low-z optical selected AGN sub-sample from Steffen et al. (2006), and our analysis shows that a constant spectral index independent of optical luminosity is also consistent with the data. By comparing X-ray and optical luminosity functions, we find that a luminosity independent spectral index is in fact more preferred than the luminosity dependent spectral index model. We also discuss the selection effects caused by flux limits, which might systematically bias the $l_x - l_o$ relation and cause discrepancy in optically selected and X-ray selected AGN samples. To correctly establish a dependence of spectral index of AGNs on their luminosity, a larger and more complete sample is needed and consequences of luminosity dispersions and selection effects in flux limited samples must be taken into account properly.Comment: 10 pages, 11 figures, accepted for publication in MNRA

Photometric variability of candidate white dwarf binary systems from Palomar Transient Factory archival data

We present a sample of 59 periodic variables from the Palomar Transient Factory, selected from published catalogues of white dwarf (WD) candidates. The variability can likely be attributed to ellipsoidal variation of the tidally distorted companion induced by the gravity of the primary (WD or hot subdwarf) or to the reflection of hot emission by a cooler companion. We searched 11311 spectroscopically or photometrically selected WD candidates from three hot star/WD catalogues, using the Lomb-Scargle periodogram to single out promising sources. We present period estimates for the candidates, 45 of which were not previously identified as periodic variables, and find that most have a period shorter than a few days. Additionally, we discuss the eclipsing systems in our sample and present spectroscopic data on selected sources

Tracing the Orphan Stream to 55 kpc with RR Lyrae Stars

We report positions, velocities and metallicities of 50 ab-type RR Lyrae (RRab) stars observed in the vicinity of the Orphan stellar stream. Using about 30 RRab stars classified as being likely members of the Orphan stream, we study the metallicity and the spatial extent of the stream. We find that RRab stars in the Orphan stream have a wide range of metallicities, from -1.5 dex to -2.7 dex. The average metallicity of the stream is -2.1 dex, identical to the value obtained by Newberg et al. (2010) using blue horizontal branch stars. We find that the most distant parts of the stream (40-50 kpc from the Sun) are about 0.3 dex more metal-poor than the closer parts (within ~30 kpc), suggesting a possible metallicity gradient along the stream's length. We have extended the previous studies and have mapped the stream up to 55 kpc from the Sun. Even after a careful search, we did not identify any more distant RRab stars that could plausibly be members of the Orphan stream. If confirmed with other tracers, this result would indicate a detection of the end of the leading arm of the stream. We have compared the distances of Orphan stream RRab stars with the best-fit orbits obtained by Newberg et al. (2010). We find that model 6 of Newberg et al. (2010) cannot explain the distances of the most remote Orphan stream RRab stars, and conclude that the best fit to distances of Orphan stream RRab stars and to the local circular velocity is provided by potentials where the total mass of the Galaxy within 60 kpc is M_{60}~2.7x10^{11} Msun, or about 60% of the mass found by previous studies. More extensive modelling that would consider non-spherical potentials and the possibility of misalignment between the stream and the orbit, is highly encouraged.Comment: Submitted to ApJ, 15 pages in emulateapj format, three tables in machine-readable format (download "Source" from "Other formats"

Molecular Targets and Associated Potential Pathways of Danlu Capsules in Hyperplasia of Mammary Glands Based on Systems Pharmacology

Hyperplasia of mammary glands (HMG) is common in middle-aged women. Danlu capsules (DLCs) can effectively relieve pain and improve clinical symptoms and are safe for treating HMG. However, the active substances in DLCs and the molecular mechanisms of DLCs in HMG remain unclear. This study identified the bioactive compounds and delineated the molecular targets and potential pathways of DLCs by using a systems pharmacology approach. The candidate compounds were retrieved from the traditional Chinese medicine systems pharmacology (TCMSP) database and analysis platform. Each candidate’s druggability was analyzed according to its oral bioavailability and drug-likeness indices. The candidate proteins and genes were extracted in the TCMSP and UniProt Knowledgebase, respectively. The potential pathways associated with the genes were identified by performing gene enrichment analysis with DAVID Bioinformatics Resources 6.7. A total of 603 compounds were obtained from DLCs, and 39 compounds and 66 targets associated with HMG were obtained. Gene enrichment analysis yielded 10 significant pathways with 34 targets. The integrated HMG pathway revealed that DLCs probably act in patients with HMG through multiple mechanisms of anti-inflammation, analgesic effects, and hormonal regulation. This study provides novel insights into the mechanisms of DLCs in HMG, from the molecular level to the pathway level

R Coronae Borealis Stars in M31 from the Palomar Transient Factory

We report the discovery of R Coronae Borealis (RCB) stars in the Andromeda galaxy (M31) using the Palomar Transient Factory (PTF). RCB stars are rare hydrogen-deficient, carbon-rich supergiant variables, most likely the merger products of two white dwarfs. These new RCBs, including two confirmed ones and two candidates, are the first to be found beyond the Milky Way and the Magellanic Clouds. All of M31 RCBs showed >1.5 mag irregular declines over timescales of weeks to months. Due to the limiting magnitude of our data (R ≈ 21-22 mag), these RCB stars have R ≈ 19.5-20.5 mag at maximum light, corresponding to M_R = –4 to –5, making them some of the most luminous RCBs known. Spectra of two objects show that they are warm RCBs, similar to the Milky Way RCBs RY Sgr and V854 Cen. We consider these results, derived from a pilot study of M31 variables, as an important proof-of-concept for the study of rare bright variables in nearby galaxies with the PTF or other synoptic surveys