Research column

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2014

Dr. Ngeow, Chow-Choong: We analyse Galactic, Large Magellanic Cloud and Small Magellanic Cloud Cepheids and RR Lyrae variables in terms of period-colour (PC) and amplitude-colour (AC) diagrams at the phases of maximum and minimum light. We compiled Galactic Cepheids V- and I-band data from the literature. We make use of optical bands light-curve data from OGLE-III survey for Cepheids and RR Lyrae variables in the Magellanic Clouds. We apply the F-statistical test to check the significance of any variation in the slope of PC and AC relations for Cepheid variables. The PC relation at maximum light for Galactic Cepheids with periods longer than about 7 d is shallow and the corresponding AC relation is flat for the entire period range. For the fundamental mode Cepheids in the Magellanic Clouds, we find significant breaks in the PC and AC relations at both maximum and minimum light for periods around 10 d. The PC relation at maximum light for the Magellanic Clouds is flat for Cepheids with periods greater than 10 d. First overtone Cepheids with periods less than 2.5 d have a shallow PC relation at minimum light. For fundamental mode RR Lyraes, we confirm earlier work supporting a flat PC relation at minimum light and a significant relation between amplitude and colour at maximum light. We find that no such relations exist for first overtone RR Lyrae stars. These findings are in agreement with stellar photosphere/hydrogen ionization front interaction considerations. These non-linearities can provide strong constraints for models of stellar pulsation and evolution.

Prof. Chou, Yi: A low mass X-ray binary (LMXB) contains either a neutron star or a black hole accreting materials from its low mass companion star. It is one of the primary astrophysical sources for studying stellar-mass compact objects and accreting phe- nomena. As with other binary systems, the most important parameter of an LMXB is the orbital period, which allows us to learn about the nature of the binary system and constrain the properties of the system’s components, including the compact ob- ject. As a result, measuring the orbital periods of LMXBs is essential for investigating these systems even though fewer than half of them have known orbital periods. This article introduces the different methods for measuring the orbital periods in the X-ray band and reviews their application to various types of LMXBs, such as eclipsing and dipping sources, as well as pulsar LMXBs. (Yi Chou, RAA, 2014, 14.1367C)

Prof. Chen, Wen-Ping: We present CCD light curves of V710 Mon obtained in 2007 and 2013. By using the 2010 version of the W-D code, it is found that the star is a deep (59.5-62.7%) contact binary system, with an extreme mass ratio (0.143-0.183). The period of the system increases with a rate dP/dt=+1.95(±0.06)×10-7 days/year. We propose that t system contains a post-main-sequence component which may be in an expanding phase. The time scale of the orbital increase nearly equals to the time for the post-main-sequence star to evolve into a subgiant. The age of the primary is estimated to be 5.34 Gyr. (L. Liu, et al. (including W. P. Chen) 2014, New Astron., 31, 60)

Dr. Chang, Chan-Kao: Asteroids of size larger than 150 m generally do not have rotation periods smaller than 2.2 hr. This spin cutoff is believed to be due to the gravitationally bound rubble-pile structures of the asteroids. Rotation with periods exceeding this critical value will cause asteroid breakup. Up until now, only one object, 2001 OE84, has been found to be an exception to this spin cutoff. We report the discovery of a new super-fast rotator, (335433) 2005 UW163, spinning with a period of 1.290 hr and a light curve variation of r' ~ 0.8 mag from the observations made at the P48 telescope and the P200 telescope of the Palomar Observatory. Its H_{r^{\prime }} = 17.69 +/- 0.27 mag and multi-band colors (i.e., g' – r' = 0.68 ± 0.03 mag, r' – i' = 0.19 ± 0.02 mag and SDSS i – z = –0.45 mag) show it is a V-type asteroid with a diameter of 0.6 + 0.3/ – 0.2 km. This indicates (335433) 2005 UW163 is a super-fast rotator beyond the regime of the small monolithic asteroid. (Chang, Chan-Kao et al., 2014, ApJL,791L,35C)

Hu, Chin-Ping:RE J1034+396, a narrow-line Seyfert-1 active galactic nucleus (AGN), is the first example of AGNs that exhibited a nearly coherent quasi-periodic oscillation (QPO) for the data collected by XMM-Newton in 2007. The spectral behaviors and timing properties of the QPO have been studied since its discovery. We present an analysis of the QPO in RE J1034+396 based on the Hilbert-Huang transform (HHT). Comparing other time-frequency analysis methods, the Hilbert spectrum reveals the variation of the QPO period in great detail. Furthermore, the empirical mode decomposition provides band-pass filtered data that can be used in the O -- C and correlation analysis. We suggest that it is better to divide the evolution of the QPO in this observation into three epochs according to their different periodicities. In addition to the periodicities, the correlations between the QPO periods and corresponding mean count rates are also different in these three epochs. Further examining the phase lags in these epochs, we found no significant phase lags between the soft and hard X-ray bands, which is also confirmed in the QPO phase-resolved spectral analysis. Finally, we discuss the indications of current models including a spotted accretion disk, diskoseismology, and oscillation of shock, according to the observed time-frequency and spectral behaviors. (Hu, Chin-Ping et al. 2014, ApJ, 788, 31)

Liu, Chin-Min: From a re-examination of the trajectories of small charged grains in the vicinity of the Saturnian rings, it is shown that the orbital motion of positively charged grains with a charge-to-mass ratio on the order of 10-6 e amu-1 could allow the injection of water material into the equatorial atmosphere. Thus, a mechanism of ionospheric depletion in the equatorial region is provided, as indicated by the Cassini radio occultation measurements. (Liu, Chin-Min & Ip, Wing-Huen 2014, ApJ, 786, 34)

Prof. Chen, Wen-Ping: Continuing the attempt to understand the properties of the stellar content in the young cluster NGC 1893 we have carried out a comprehensive multi-wavelength study of the region. The present study focuses on the X-ray properties of T-Tauri Stars (TTSs) in the NGC 1893 region. We found a correlation between the X-ray luminosity, LX, and the stellar mass (in the range 0.2-2.0 M) of TTSs in the NGC 1893 region, similar to those reported in some other young clusters, however the value of the power-law slope obtained in the present study (∼0.9) for NGC 1893 is maller than those (∼1.4-3.6) reported in the case of TMC, ONC, IC 348 and Chameleon star forming regions. Howeve, the slope in the case of Class III sources (Weak line TTSs) is found to be comparable to that reported in the case of NGC 6611 (∼1.1). It is found that the presence of circumstellar disks has no influence on the X-ray emisson. The X-ray luminosity for both CTTSs and WTTSs is found to decrease systematically with age (in the range ∼0.-5 Myr). The decrease of the X-ray luminosity of TTSs (slope ∼-0.6) in the case of NGC 1893 seems to be faster tan observed in the case of other star-forming regions (slope -0.2 to -0.5). There is indication that the sources having relatively large NIR excess have relatively lower LX values. TTSs in NGC 1893 do not follow the well established X-ray activity - rotation relation as in the case of main-sequence stars. (Pandey, A.K. et al. (including W. P. Chen) 2014, New Astron., 29, 18)

Dr. Lin, Hsing-Wen: P/2011 S1 (Gibbs) is an outer solar system comet or active Centaur with a similar orbit to that of the famous 29P/Schwassmann-Wachmann 1. P/2011 S1 (Gibbs) has been observed by the Pan-STARRS 1 (PS1) sky survey from 2010 to 2012. The resulting data allow us to perform multi-color studies of the nucleus and coma of the comet. Analysis of PS1 images reveals that P/2011 S1 (Gibbs) has a small nucleus less than 4 km radius, with colors g P1 – r P1 =0.5 ± 0.02, r P1 – i P1 = 0.12 ± 0.02, and i P1 – z P1 = 0.46 ± 0.03. The comet remained active from 2010 to, with a model-dependent mass-loss rate of ~100 kg s–1. The mass-loss rate per unit surface area of P/2011 S1 (Gbbs) is as high as that of 29P/Schwassmann-Wachmann 1, making it one of the most active Centaurs. The mass-loss rate also varies with time from ~40 kg s–1 to 150 kg s–1. Due to its rather circular orbit, we propose that P/20 S1 (Gibbs) has 29P/Schwassmann-Wachmann 1-like outbursts that control the outgassing rate. The results indicate that it may have a similar surface composition to that of 29P/Schwassmann-Wachmann 1. Our numerical simulations show that the future orbital evolution of P/2011 S1 (Gibbs) is more similar to that of the main population of Centaurs than to that of 29P/Schwassmann-Wachmann 1. The results also demonstrate that P/2011 S1 (Gibbs) is dynamically unstable and can only remain near its current orbit for roughly a thousand years. (Lin, Hsing-Wen et al. 2014, AJ, 147, 114)

Dr. Ngeow, Chow-Choong: The Taiwanese-American Occultation Survey project is designed for the detection of stellar occultations by small-size Kuiper Belt Objects, and it has monitored selected fields along the ecliptic plane by using four telescopes with a 3 deg2 field of view on the sky since 2005. We have analyzed data accumulated during 2005-2012 to detect variable stars. Sixteen fields with observations of more than 100 epochs were examined. We recovered 85 variables among a total of 158 known variable stars in these 16 fields. Most of the unrecovered variables are located in the fields observed less frequently. We also detected 58 variable stars which are not listed in the International Variable Star Index of the American Association of Variable Star Observers. These variable stars are classified as 3 RR Lyrae, 4 Cepheid, 1 δ Scuti, 5 Mira, 15 semi-regular, and 27 eclipsing binaries based on the periodicty and the profile of the light curves. (Ishioka, R. et al. (Including: Ngeow,C.-C., Chen,Wen-Ping) 2014, AJ, 147, 70I)

Lin, Chien-Cheng: By using the 2MASS data, we identified a star cluster G144.9+0.4 which is physically associated with the Cam OB1 association at a distance of about 1 kpc and with an age of 1-2 Myr. The cluster members including pre-main sequence stars are identified on the basis of photometric and proper motion data. A total of 91 additional OB star candidates was found in Cam OB 1A and 1B. The OB members show an age spread that indicates a sustained star formation for at least the last 10-15 Myr. The young cluster G144.9+0.4 represents the latest episode of sequential star formation in this cloud complex. (Lin, Chien-Cheng et al. 2013, ApJ, 775, 123)

Prof. Chen, Wen-Ping: Membership identification is the first step in determining the properties of a star cluster. Low-mass members in particular could be used to trace the dynamical history, such as mass segregation, stellar evaporation, or tidal stripping, of a star cluster in its Galactic environment. We identified member candidates of the intermediate-age Praesepe cluster (M44) with stellar masses ~0.11-2.4 M ⊙, using Panoramic Survey Telescope And Rapid ResponseSystem and Two Micron All Sky Survey photometry, and PPMXL proper motions. Within a sky area of 3° radius, 1040 andidates are identified, of which 96 are new inclusions. Using the same set of selection criteria on field stars, an estimated false positive rate of 16% was determined, suggesting that 872 of the candidates are true members. This most complete and reliable membership list allows us to favor the BT-Settl model over other stellar models. The cluster shows a distinct binary track above the main sequence, with a binary frequency of 20%-40%, and a high occurrence rate of similar mass pairs. The mass function is consistent with that of the disk population but shows a deficit of members below 0.3 solar masses. A clear mass segregation is evidenced, with the lowest-mass members in our sample being evaporated from this disintegrating cluster. (P. F. Wang et al. (including W. P. Chen) 2014, ApJ, 784, 57)

Prof. Chen, Wen-Ping: The Cerenkov line-like radiation in a dense gas (NH > 1013 cm-3) is potentially important in the exploration of the optical broad emission lines of quasars and Seyfert 1 galaxies. With this quasi-line emission mechanism, some long standing puzzles in the study of quasars could be resolved. In this paper, we calculate the power of the Cerenkov line-like radiation in dense gas and compare with the powers of other radiation mechanisms by a fast electron to confirm its importance. From the observed gamma-ray luminosity of 3C 279, we show that the total number of fast electrons is sufficiently high to allow effective operation of the quasi-line emission. We present a model calculation for the luminosity of the Cerenkov Lyα line of 3C 279, which is hgh enough to compare with observations. We therefore conclude that the broad line of quasars may be a blend of the Cerenkov emission line with the real line produced by the bound-bound transition. A new approach to the absorption of the Cerenkov line is presented with the method of escape probability, which markedly simplifies the computation in the optically thick case. The revised set of formulae for the Cerenkov line-like radiation is more convenient in applications. (D. B. Liu et al.(including W. P. Chen) 2014, ApJ, 780, 89)