arXiv subjects

W. P. Chen

Publications and source records attributed to W. P. Chen.

At least 55 records · Page 3Linked to original sources

arXiv2014

Characterization of the Praesepe Star Cluster by Photometry and Proper Motions with 2MASS, PPMXL, and Pan-STARRS

Membership identification is the first step to determine 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 with stellar masses $\sim$0.11--2.4 Msun of the intermediate-age Praesepe cluster (M44), by using Pan-STARRS and 2MASS photometry, and PPMXL proper motions. Within a sky area of 3 deg radius, 1040 candidates are identified, of which 96 are new inclusions. Using the same set of selection criteria on field stars led to an estimate of a false positive rate 16%, suggesting 872 of the candidates being true members. This most complete and reliable membership list allows us to favor the BT-Settl model in comparison with 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.

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arXiv2013

Pre-main-sequence population in NGC 1893 region: X-ray properties

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, $L_X$, and the stellar mass (in the range 0.2$-$2.0 \msun) of TTSs in the NGC 1893 region, similar to those reported in some other young clusters, however the value of the power-law slope obtained in the present study ($\sim$ 0.9) for NGC 1893 is smaller than those ($\sim$1.4 - 3.6) reported in the case of TMC, ONC, IC 348 and Chameleon star forming regions. However, the slope in the case of Class III sources (Weak line TTSs) is found to be comparable to that reported in the case of NGC 6611 ($\sim$ 1.1). It is found that the presence of circumstellar disks has no influence on the X-ray emission. The X-ray luminosity for both CTTSs and WTTSs is found to decrease systematically with age (in the range $\sim $ 0.4 Myr - 5 Myr). The decrease of the X-ray luminosity of TTSs (slope $\sim$ -0.6) in the case of NGC 1893 seems to be faster than 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 $L_X$ values. TTSs in NGC 1893 do not follow the well established X-ray activity - rotation relation as in the case of main-sequence stars.

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arXiv2013

Characterization of the Young Open Cluster G144.9+0.4 in the Camelopardalis OB1 Association

Our star-count analysis of the Two Micron All Sky Survey point sources resulted in an identification of the star cluster G144.9+0.4. The cluster was found, but not characterized, by Glushkova et al. We show that the cluster is physically associated with the Cam OB1 association at a distance of about 1 kpc and with an age of 1-2 Myr. 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 subgroups 1A and 1B, a significant increase from the currently known 43 OB stars. 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.

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arXiv2013

Search for and Characterization of Open Clusters Toward the Galactic Anti-center with Pan-STARRS1

We have used a star-count algorithm based the Panoramic Survey Telescope And Rapid Response System 3$π$ survey data aim to identify and characterize uncharted open clusters (OCs). With limiting magnitudes of about 22 mag in \gps, \rps, \ips\ bands and about 20 mag in \zps\ and \yps\ bands, our data are 100 times more sensitive than currently available surveys. We analyzed a trial region within $20°\times20°$ field toward the Galactic anticenter and found 1660 density enhancement regions of which 79 (out of 129) are known OCs, and 949 are OC candidates.

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arXiv2013

Towards a complete stellar mass function of the Hyades. I. Pan-STARRS1 optical observations of the low-mass stellar content

The Hyades cluster is an ideal target to study the dynamical evolution of a star cluster over the entire mass range due to its intermediate age and proximity to the Sun. We wanted to extend the Hyades mass function towards lower masses down to 0.1 Msol and to use the full three-dimensional spatial information to characterize the dynamical evolution of the cluster. We performed a kinematic and photometric selection using the PPMXL and Pan-STARRS1 sky surveys, to search for cluster members up to 30 pc from the cluster centre. We determined our detection efficiency and field star contamination rate to derive the cluster luminosity and mass functions down to masses of 0.1 Msol. The thorough astrometric and photometric constraints minimized the contamination. A minimum spanning tree algorithm was used to quantify the mass segregation. We discovered 43 new Hyades member candidates with velocity perpendicular to the Hyades motion up to 2 km/s. They have mass estimates between 0.43 and 0.09 Msol, for a total mass of 10 Msol. This doubles the number of Hyades candidates with masses smaller than 0.15 Msol. We provide an additional list of 11 possible candidates with velocity perpendicular to the Hyades motion up to 4 km/s. The cluster is significantly mass segregated. The extension of the mass function towards lower masses provided an even clearer signature than estimated in the past. We also identified as likely Hyades member an L0 dwarf previously assumed to be a field dwarf. Finally we question the membership of a number of previously published candidates, including a L2.5-type dwarf.

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arXiv2013

The stellar content of the young open cluster Trumpler 37

With an apparent cluster diameter of 1.5° and an age of ~4 Myr, Trumpler 37 is an ideal target for photometric monitoring of young stars as well as for the search of planetary transits, eclipsing binaries and other sources of variability. The YETI consortium has monitored Trumpler 37 throughout 2010 and 2011 to obtain a comprehensive view of variable phenomena in this region. In this first paper we present the cluster properties and membership determination as derived from an extensive investigation of the literature. We also compared the coordinate list to some YETI images. For 1872 stars we found literature data. Among them 774 have high probability of being member and 125 a medium probability. Based on infrared data we re-calculate a cluster extinction of 0.9-1.2 mag. We can confirm the age and distance to be 3-5 Myr and ~870 pc. Stellar masses are determined from theoretical models and the mass function is fitted with a power-law index of alpha=1.90 (0.1-0.4 M_sun) and alpha=1.12 (1-10 M_sun).

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arXiv2013

Multi-site campaign for transit timing variations of WASP-12 b: possible detection of a long-period signal of planetary origin

The transiting planet WASP-12 b was identified as a potential target for transit timing studies because a departure from a linear ephemeris was reported in the literature. Such deviations could be caused by an additional planet in the system. We attempt to confirm the existence of claimed variations in transit timing and interpret its origin. We organised a multi-site campaign to observe transits by WASP-12 b in three observing seasons, using 0.5-2.6-metre telescopes. We obtained 61 transit light curves, many of them with sub-millimagnitude precision. The simultaneous analysis of the best-quality datasets allowed us to obtain refined system parameters, which agree with values reported in previous studies. The residuals versus a linear ephemeris reveal a possible periodic signal that may be approximated by a sinusoid with an amplitude of 0.00068+/-0.00013 d and period of 500+/-20 orbital periods of WASP-12 b. The joint analysis of timing data and published radial velocity measurements results in a two-planet model which better explains observations than single-planet scenarios. We hypothesize that WASP-12 b might be not the only planet in the system and there might be the additional 0.1 M_Jup body on a 3.6-d eccentric orbit. A dynamical analysis indicates that the proposed two-planet system is stable over long timescales.

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arXiv2013

The TAOS Project: Results From Seven Years of Survey Data

The Taiwanese-American Occultation Survey (TAOS) aims to detect serendipitous occultations of stars by small (about 1 km diameter) objects in the Kuiper Belt and beyond. Such events are very rare (<0.001 events per star per year) and short in duration (about 200 ms), so many stars must be monitored at a high readout cadence. TAOS monitors typically around 500 stars simultaneously at a 5 Hz readout cadence with four telescopes located at Lulin Observatory in central Taiwan. In this paper, we report the results of the search for small Kuiper Belt Objects (KBOs) in seven years of data. No occultation events were found, resulting in a 95% c.l. upper limit on the slope of the faint end of the KBO size distribution of q = 3.34 to 3.82, depending on the surface density at the break in the size distribution at a diameter of about 90 km.

astro-ph.EP↗

arXiv2012

Optical photometric and polarimetric investigation of NGC 1931

We present optical photometric and polarimetric observations of stars towards NGC 1931 with the aim to derive the cluster parameters such as distance, reddening, age and luminosity/mass function as well as to understand the dust properties and star formation in the region. The distance to the cluster is found to be 2.3$\pm$0.3 kpc and the reddening E(B-V) in the region is found to be variable. The stellar density contours reveal two clustering in the region. The observations suggest differing reddening law within the cluster region. Polarization efficiency of the dust grains towards the direction of the cluster is found to be less than that for the general diffuse interstellar medium (ISM). The slope of the mass function (-0.98$\pm$0.22) in the southern region in the mass range 0.8 \textless $M/M_{\sun}$ $\textless$ 9.8 is found to be shallower in comparison to that in the northern region (-1.26$\pm$0.23), which is comparable to the Salpeter value (-1.35). The K-band luminosity function (KLF) of the region is found to be comparable to the average value of slope (~0.4) for young clusters obtained by Lada & Lada (2003), however, the slope of the KLF is steeper in the northern region as compared to the southern region. The region is probably ionized by two B2 main-sequence type stars. The mean age of the young stellar objects (YSOs) is found to be 2$\pm$1 Myr which suggests that the identified YSOs could be younger than the ionizing sources of the region. The morphology of the region, the distribution of the YSOs as well as ages of the YSOs and ionizing sources indicate a triggered star formation in the region.

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arXiv2012

Pre-main sequence variable stars in young open cluster NGC 1893

We present results of multi-epoch (fourteen nights during 2007-2010) $V$-band photometry of the cluster NGC 1893 region to identify photometric variable stars in the cluster. The study identified a total of 53 stars showing photometric variability. The members associated with the region are identified on the basis of spectral energy distribution, $J-H/H-K$ two colour diagram and $V/V-I$ colour-magnitude diagram. The ages and masses of the majority of pre-main-sequence sources are found to be $\lesssim$ 5 Myr and in the range 0.5 $\lesssim$ $M/M_{\odot}$ $\lesssim$ 4, respectively. These pre-main-sequence sources hence could be T Tauri stars. We also determined the physical parameters like disk mass and accretion rate from the spectral energy distribution of these T Tauri stars. The periods of majority of the T Tauri stars range from 0.1 to 20 day. The brightness of Classical T Tauri stars is found to vary with larger amplitude in comparison to Weak line T Tauri stars. It is found that the amplitude decreases with increase in mass, which could be due to the dispersal of disks of massive stars.

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arXiv2012

Pre-main-sequence population in NGC 1893 region

In this paper we continued our efforts to understand the star formation scenario in and around the young cluster NGC 1893. We used a sample of the young stellar sources (YSOs) identified on the basis of multiwavelength data (optical, near-infrared (NIR), mid-infrared (MIR) and X-ray) to study the nature of YSOs associated with the region. The identified YSOs show an age spread of ~ 5 Myr. The YSOs located near the nebulae at the periphery of the cluster are relatively younger in comparison to those located within the cluster region. The present results are in accordance with those obtained by us in previous studies. Other main results from the present study are: 1) the fraction of disk bearing stars increases towards the periphery of the cluster; 2) there is an evidence supporting the notion that the mechanisms for disk dispersal operate less efficiently for low-mass stars; 3) the sample of Class II sources is found to be relatively older in comparison to that of Class III sources. A comparison of various properties of YSOs in the NGC 1893 region with those in the Tr 37/ IC 1396 region is also discussed.

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arXiv2012

Variability of the blazar 4C 38.41 (B3 1633+382) from GHz frequencies to GeV energies

The quasar-type blazar 4C 38.41 (B3 1633+382) experienced a large outburst in 2011, which was detected throughout the entire electromagnetic spectrum. We present the results of low-energy multifrequency monitoring by the GASP project of the WEBT consortium and collaborators, as well as those of spectropolarimetric/spectrophotometric monitoring at the Steward Observatory. We also analyse high-energy observations of the Swift and Fermi satellites. In the optical-UV band, several results indicate that there is a contribution from a QSO-like emission component, in addition to both variable and polarised jet emission. The unpolarised emission component is likely thermal radiation from the accretion disc that dilutes the jet polarisation. We estimate its brightness to be R(QSO) ~ 17.85 - 18 and derive the intrinsic jet polarisation degree. We find no clear correlation between the optical and radio light curves, while the correlation between the optical and γ-ray flux apparently fades in time, likely because of an increasing optical to γ-ray flux ratio. As suggested for other blazars, the long-term variability of 4C 38.41 can be interpreted in terms of an inhomogeneous bent jet, where different emitting regions can change their alignment with respect to the line of sight, leading to variations in the Doppler factor δ. Under the hypothesis that in the period 2008-2011 all the γ-ray and optical variability on a one-week timescale were due to changes in δ, this would range between ~ 7 and ~ 21. If the variability were caused by changes in the viewing angle θ only, then θ would go from ~ 2.6 degr to ~ 5 degr. Variations in the viewing angle would also account for the dependence of the polarisation degree on the source brightness in the framework of a shock-in-jet model.

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arXiv2012

The structure and emission model of the relativistic jet in the quasar 3C 279 inferred from radio to high-energy gamma-ray observations in 2008-2010

We present time-resolved broad-band observations of the quasar 3C 279 obtained from multi-wavelength campaigns conducted during the first two years of the Fermi Gamma-ray Space Telescope mission. While investigating the previously reported gamma-ray/optical flare accompanied by a change in optical polarization, we found that the optical emission appears delayed with respect to the gamma-ray emission by about 10 days. X-ray observations reveal a pair of `isolated' flares separated by ~90 days, with only weak gamma-ray/optical counterparts. The spectral structure measured by Spitzer reveals a synchrotron component peaking in the mid-infrared band with a sharp break at the far-infrared band during the gamma-ray flare, while the peak appears in the mm/sub-mm band in the low state. Selected spectral energy distributions are fitted with leptonic models including Comptonization of external radiation produced in a dusty torus or the broad-line region. Adopting the interpretation of the polarization swing involving propagation of the emitting region along a curved trajectory, we can explain the evolution of the broad-band spectra during the gamma-ray flaring event by a shift of its location from ~ 1 pc to ~ 4 pc from the central black hole. On the other hand, if the gamma-ray flare is generated instead at sub-pc distance from the central black hole, the far-infrared break can be explained by synchrotron self-absorption. We also model the low spectral state, dominated by the mm/sub-mm peaking synchrotron component, and suggest that the corresponding inverse-Compton component explains the steady X-ray emission.

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arXiv2012

Time delay between images of the lensed quasar UM673

We study brightness variations in the double lensed quasar UM673 (Q0142-100) with the aim of measuring the time delay between its two images. In the paper we combine our previously published observational data of UM673 obtained during the 2003 - 2005 seasons at the Maidanak Observatory with archival and recently observed Maidanak and CTIO UM673 data. We analyze the V, R and I-band light curves of the A and B images of UM673, which cover ten observational seasons from August 2001 to November 2010. We also analyze the time evolution of the difference in magnitudes between images A and B of UM673 over more than ten years. We find that the quasar exhibits both short-term (with amplitude of \sim 0.1 mag in the R band) and high-amplitude (\sim 0.3 mag) long-term variability on timescales of about several months and several years, respectively. These brightness variations are used to constrain the time delay between the images of UM673. From cross-correlation analysis of the A and B quasar light curves and error analysis we measure the mean time delay and its error of 89 \pm11 days. Given the input time delay of 88 days, the most probable value of the delay that can be recovered from light curves with the same statistical properties as the observed R-band light curves of UM673 is 95{+5/-16}{+14/-29} days (68 and 95 % confidence intervals). Analysis of the V - I color variations and V, R and I-band magnitude differences of the quasar images does not show clear evidence of the microlensing variations between 1998 and 2010.

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arXiv2012

A Possible Detection of Occultation by a Proto-planetary Clump in GM Cephei

GM Cep in the young (~4 Myr) open cluster Trumpler 37 has been known to be an abrupt variable and to have a circumstellar disk with very active accretion. Our monitoring observations in 2009-2011 revealed the star to show sporadic flare events, each with brightening of < 0.5 mag lasting for days. These brightening events, associated with a color change toward the blue, should originate from an increased accretion activity. Moreover, the star also underwent a brightness drop of ~1 mag lasting for about a month, during which the star became bluer when fainter. Such brightness drops seem to have a recurrence time scale of a year, as evidenced in our data and the photometric behavior of GM Cep over a century. Between consecutive drops, the star brightened gradually by about 1 mag and became blue at peak luminosity. We propose that the drop is caused by obscuration of the central star by an orbiting dust concentration. The UX Orionis type of activity in GM Cep therefore exemplifies the disk inhomogeneity process in transition between grain coagulation and planetesimal formation in a young circumstellar disk.

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arXiv2012

Photometric Observations of the Young Cluster Variable GM Cephei

We present our photometric observations of GM Cep, a solar type variable in the young (~4 Myr) open cluster Trumpler 37. The star is known as a classical T Tauri star with a circumstellar disk and active accretion. GM\,Cep was suspected to undergo an outburst, thus a candidate for an EXor-type variable. In our monitoring campaign observations in 2010-2011, GM Cep experienced a ~0.82 mag brightness decrease in the R band lasting for 39 days, and frequent, transient flare-like episodes with amplitude < 1 mag, each lasting for about 10 days. The brightening was accompanied with a bluer color, presumably arising from increased accretion activity. Interestingly, the star also turned bluer in the fading phase. Combining the AAVSO and literature data, we found a quasi-cyclic peroid of ~311 days for the fading event. A possible mechanism for the fading could be obscuration by a clump of dust around the star. We proposed that GM Cep therefore should be a UXor-type variable in the transition phase between grain coagulation and planetesimal formation process in the circumstellar disk.

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arXiv2012

Search for and Characterization of Galactic Open Clusters with 2MASS

We have developed a star-counting algorithm and tested it on the 2MASS star catalog to search for density enhancements significantly above the field in Galactic latitude $|b|<50^{\circ}$. Nearly 500 open clusters are "rediscovered", along with 89 globular clusters, 35 galaxies, 55 galaxy clusters, 11 H II regions, and 4 regions contaminated by nearby bright stars. Fifty-two density enhancement regions remain unaccounted for. Here we present one such candidate HDG\,01, $(\ell, b)=(144.9038, 0.4338)$ which has an angular size of 3', a distance of 1.5 kpc, hence a physical size of 3.7 pc. Due to some nebulous shape, this star cluster should be an young-aged ($\le 10$ Myr).

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arXiv2011

Broad-band Optical Polarimetric Studies toward the Galactic young star cluster Be 59

We present multiwavelength optical linear polarimetric observations of 69 stars toward the young open cluster Be 59. The observations reveal the presence of three dust layers located at the distances of \sim300, \sim500 and \sim700 pc. The dust layers produce a total polarization Pv \sim 5.5 per cent. The mean values of polarization and polarization angles due to the dust layers are found to increase systematically with distance. We show that polarimetry in combination with the (U - B) - (B - V) colour-colour diagram yields a better identification of cluster members. The polarization measurements suggest that the polarization due the intra-cluster medium is \sim 2.2 per cent. An anomalous reddening law exists for the cluster region, indicating a relatively larger grain size than that in the diffuse ISM. The spatial variation of the polarization and E(B - V) is found to increase with radial distance from the cluster center, whereas the θv and λmax are found to decrease with increasing radial distance from the cluster center. About 40 per cent of cluster members show the signatures of either intrinsic polarization or rotation in their polarization angles. There is an indication that the star light of the cluster members might have been depolarized because of non-uniform alignment of dust grains in the foreground dust layers and in the intra-cluster medium.

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