A Systematic Study of X-Ray Flares from Low-Mass Young Stellar Objects in the Rho Ophiuchi Star-Forming Region with Chandra

We report on the results of a systematic study of X-ray flares from low-mass young stellar objects, using Chandra observations of the main region of the Rho Oph. From 195 X-ray sources, including class I-III sources and some young brown dwarfs, we detected a total of 71 X-ray flares. Most of the flares have the typical profile of solar and stellar flares, fast rise and slow decay. We derived the time-averaged temperature (kT), luminosity (L_X), rise and decay timescales (tau_r and tau_d) of the flares, finding that (1) class I-II sources tend to have a high kT, (2) the distribution of L_X during flares is nearly the same for all classes, and (3) positive and negative log-linear correlations are found between tau_r and tau_d, and kT and tau_r. In order to explain these relations, we used the framework of magnetic reconnection model to formulate the observational parameters as a function of the half-length of the reconnected magnetic loop (L) and magnetic field strength (B). The estimated L is comparable to the typical stellar radius of these objects (10^{10-11} cm), which indicates that the observed flares are triggered by solar-type loops, rather than larger ones (10^{12} cm) connecting the star with its inner accretion disk. The higher kT observed for class I sources may be explained by a higher magnetic field strength (about 500 G) than for class II-III sources (200-300 G).Comment: 33 pages, 7 figures, accepted for publication in PASJ, the complete version of tables are available at ftp://ftp-cr.scphys.kyoto-u.ac.jp/pub/crmember/kensuke/PASJ_RhoOph/KI_all.tar .g

Detections of water ice, hydrocarbons, and 3.3um PAH in z~2 ULIRGs

We present the first detections of the 3um water ice and 3.4um amorphous hydrocarbon (HAC) absorption features in z~2 ULIRGs. These are based on deep rest-frame 2-8um Spitzer IRS spectra of 11 sources selected for their appreciable silicate absorption. The HAC-to-silicate ratio for our z~2 sources is typically higher by a factor of 2-5 than that observed in the Milky Way. This HAC `excess' suggests compact nuclei with steep temperature gradients as opposed to predominantly host obscuration. Beside the above molecular absorption features, we detect the 3.3um PAH emission feature in one of our sources with three more individual spectra showing evidence for it. Stacking analysis suggests that water ice, hydrocarbons, and PAH are likely present in the bulk of this sample even when not individually detected. The most unexpected result of our study is the lack of clear detections of the 4.67um CO gas absorption feature. Only three of the sources show tentative signs of this feature and at significantly lower levels than has been observed in local ULIRGs. Overall, we find that the closest local analogs to our sources, in terms of 3-4um color, HAC-to-silicate and ice-to-silicate ratios, as well as low PAH equivalent widths are sources dominated by deeply obscured nuclei. Such sources form only a small fraction of ULIRGs locally and are commonly believed to be dominated by buried AGN. Our sample suggests that, in absolute number, such buried AGN are at least an order of magnitude more common at z~2 than today. The presence of PAH suggests that significant levels of star-formation are present even if the obscured AGN typically dominate the power budget.Comment: 39 pages, 14 figures, accepted for publication in Ap

The nature of the fluorescent iron line in V 1486 Ori

The fluorescent 6.4 keV iron line provides information on cool material in the vicinity of hard X-ray sources as well as on the characteristics of the X-ray sources themselves. First discovered in the X-ray spectra of the flaring Sun, X-ray binaries and active galactic nuclei (AGN), the fluorescent line was also observed in a number of stellar X-ray sources. The young stellar object (YSO) V1486 Ori was observed in the framework of the Chandra Ultra Deep Project (COUP) as the source COUP 331. We investigate its spectrum, with emphasis on the strength and time variability of the fluorescent iron K-alpha line, derive and analyze the light curve of COUP 331 and proceed with a time-resolved spectral analysis of the observation. The light curve of V 1486 Ori shows two major flares, the first one lasting for (approx) 20 ks with a peak X-ray luminosity of 2.6*10^{32} erg/s (dereddened in the 1-10 keV band) and the second one -- only partially observed -- for >60 ks with an average X-ray luminosity of 2.4*10^{31} erg/s (dereddened). The spectrum of the first flare is very well described by an absorbed thermal model at high temperature, with a pronounced 6.7 keV iron line complex, but without any fluorescent K-alpha line. The X-ray spectrum of the second flare is characterized by even higher temperatures (>= 10 keV) without any detectable 6.7 keV Fe XXV feature, but with a very strong fluorescent iron K-alpha line appearing predominantly in the 20 ks rise phase of the flare. Preliminary model calculations indicate that photoionization is unlikely to account for the entire fluorescent emission during the rise phase.Comment: 4 pages, letter, accepted for publication in A&

X-ray Detections of Two Young Bona-Fide Brown Dwarfs

I report here the detection of two bona-fide brown dwarfs by XMM-Newton: [GY92] 141 in the rho-Ophiuchus star forming region and DENIS-P J155601-233809 in the Upper Scorpius OB association. The two objects have been detected with luminosities of Lx=8.35\pm2.86*10^28 erg/s and Lx=6.54\pm1.35*10^28 erg/s respectively, corresponding to luminosity ratios of log(Lx/Lbol)= -2.07 and log(Lx/Lbol)= -2.69. The two sources were close to the limit of detection of the instruments at only 2~3-sigma above the background level, and no significant flare or variation could be detected during the 48.3ks and 33.9ks observations. [GY92] 141 had already been observed 10 months earlier with Chandra (see 2001 ApJ, 563, 361) with a luminosity ~14 times fainter than the one I report here, meaning that the X-ray emission of this object is strongly variable.Comment: 9 pages, 3 figures, accepted for A&

The X-ray emission from Young Stellar Objects in the rho Ophiuchi cloud core as seen by XMM-Newton

We observed the main core F of the rho Ophiuchi cloud, an active star-forming region located at ~140 pc, using XMM-Newton with an exposure of 33 ks. We detect 87 X-ray sources within the 30' diameter field-of-view of the it EPIC imaging detector array. We cross-correlate the positions of XMM-Newton X-ray sources with previous X-ray and infrared (IR) catalogs: 25 previously unknown X-ray sources are found from our observation; 43 X-ray sources are detected by both XMM-Newton and Chandra; 68 XMM-Newton X-ray sources have 2MASS near-IR counterparts. We show that XMM-Newton and Chandra have comparable sensitivity for point source detection when the exposure time is set to ~30 ks for both. We detect X-ray emission from 7 Class I sources, 26 Class II sources, and 17 Class III sources. The X-ray detection rate of Class I sources is very high (64 %), which is consistent with previous Chandra observations in this area. We propose that 15 X-ray sources are new class III candidates, which doubles the number of known Class III sources, and helps to complete the census of YSOs in this area. We also detect X-ray emission from two young bona fide brown dwarfs, GY310 and GY141, out of three known in the field of view. GY141 appears brighter by nearly two orders of magnitude than in the Chandra observation. We extract X-ray light curves and spectra from these YSOs, and find some of them showed weak X-ray flares. We observed an X-ray flare from the bona fide brown dwarf GY310. We find as in the previous Chandra observation of this region that Class I sources tend to have higher temperatures and heavier X-ray absorptions than Class II and III sources.Comment: 17 pages, 13 figures, 4 tables, accepted by A&

A Radio Perspective on the Wet Merger Remnant NGC 34

We present VLA observations of the neutral hydrogen and radio continuum of NGC 34 (= NGC 17 = Mrk 938). This object is an ideal candidate to study the fate of gas in mergers, since, as shown by an optical study done by Schweizer & Seitzer (2007), it is a gas-rich ("wet") merger remnant of two disk galaxies of unequal mass hosting a strong central starburst and a weak AGN. We detect HI emission from both tidal tails and from nearby galaxies, suggesting that NGC 34 is actually part of a gas-rich group and might have recently interacted with one of its companions. The kinematics of the gas suggests this remnant is forming an outer disk of neutral hydrogen from the gas of the northern tail. We also detect broad HI absorption (514 +/- 21 km/s wide) at both negative and positive velocities with respect to the systemic velocity. This absorption could be explained by the motions of the tidal tails or by the presence of a circumnuclear disk. In addition, we present radio-continuum images that show both nuclear (62.4 +/- 0.3 mJy) and extra-nuclear emission (26.5 +/- 3.0 mJy). The extra-nuclear component is very diffuse and in the shape of two radio lobes, spanning 390 kpc overall. This emission could be a signature of an AGN that has turned off, or it could originate from a starburst-driven superwind. We discuss the possible scenarios that explain our observations, and what they tell us about the location of the gas and the future evolution of NGC 34.Comment: 29 pages, 12 figures, 2 tables. Accepted for publication in the Astronomical Journal. Figs. 1, 2 & 6 degraded to reduce file size

Infrared 3-4 Micron Spectroscopic Investigations of a Large Sample of Nearby Ultraluminous Infrared Galaxies

We present infrared L-band (3-4 micron) nuclear spectra of a large sample of nearby ultraluminous infrared galaxies (ULIRGs).ULIRGs classified optically as non-Seyferts (LINERs, HII-regions, and unclassified) are our main targets. Using the 3.3 micron polycyclic aromatic hydrocarbon (PAH) emission and absorption features at 3.1 micron due to ice-covered dust and at 3.4 micron produced by bare carbonaceous dust, we search for signatures of powerful active galactic nuclei (AGNs) deeply buried along virtually all lines-of-sight. The 3.3 micron PAH emission, the signatures of starbursts, is detected in all but two non-Seyfert ULIRGs, but the estimated starburst magnitudes can account for only a small fraction of the infrared luminosities. Three LINER ULIRGs show spectra typical of almost pure buried AGNs, namely, strong absorption features with very small equivalent-width PAH emission. Besides these three sources, 14 LINER and 3 HII ULIRGs' nuclei show strong absorption features whose absolute optical depths suggest an energy source more centrally concentrated than the surrounding dust, such as a buried AGN. In total, 17 out of 27 (63%) LINER and 3 out of 13 (23%) HII ULIRGs' nuclei show some degree of evidence for powerful buried AGNs, suggesting that powerful buried AGNs may be more common in LINER ULIRGs than in HII ULIRGs. The evidence of AGNs is found in non-Seyfert ULIRGs with both warm and cool far-infrared colors. These spectra are compared with those of 15 ULIRGs' nuclei with optical Seyfert signatures taken for comparison.The overall spectral properties suggest that the total amount of dust around buried AGNs in non-Seyfert ULIRGs is systematically larger than that around AGNs in Seyfert 2 ULIRGs.Comment: 56 pages, 9 figures, accepted for publication in ApJ (20 January 2006, vol 637 issue

Hyperlink Management System and ID Converter System: enabling maintenance-free hyperlinks among major biological databases

Hyperlink Management System (HMS) is a system for automatically updating and maintaining hyperlinks among major public databases in the field of life science. We daily create corresponding tables of data IDs of major databases for human genes and proteins, and provide a CGI-program that returns correct and up-to-date URLs for showing data of various databases that correspond to user-specified IDs. The HMS can deal with various IDs: accession numbers of International Nucleotide Sequence Databases, HUGO Gene Symbols and IDs of UniProt, PDB, H-InvDB and others, and it can return URLs of various databases: H-InvDB, HUGO Gene Nomenclature Committee Database, NCBI Entrez Gene, UniProt, PDB and others. For example, 23 297 pages of Locus view of H-InvDB are reachable by using HUGO Gene Symbols through the HMS. Not only the CGI-program, the HMS provides a Web page for finding and opening URLs of these databases. Although hyperlinking is an effective way of relating biological data among different databases, updating hyperlinks has been a laborious work. The HMS fully automates the job, enabling maintenance-free hyperlinks. We also developed the ID Converter System (ICS) for simply converting data IDs by using corresponding tables in the HMS. The HMS and ICS are freely available at http://biodb.jp/

Xk-Related Protein 8 and CED-8 Promote Phosphatidylserine Exposure in Apoptotic Cells

アポトーシス時のリン脂質暴露に関与する因子の同定. 京都大学プレスリリース. 2013-07-12.A classic feature of apoptotic cells is the cell-surface exposure of phosphatidylserine (PtdSer) as an "eat me" signal for engulfment. We show that the Xk-family protein Xkr8 mediates PtdSer exposure in response to apoptotic stimuli. Mouse Xkr8(-/-) cells or human cancer cells in which Xkr8 expression was repressed by hypermethylation failed to expose PtdSer during apoptosis and were inefficiently engulfed by phagocytes. Xkr8 was activated directly by caspases and required a caspase-3 cleavage site for its function. CED-8, the only Caenorhabditis elegans Xk-family homolog, also promoted apoptotic PtdSer exposure and cell-corpse engulfment. Thus, Xk-family proteins have evolutionarily conserved roles in promoting the phagocytosis of dying cells by altering the phospholipid distribution in the plasma membrane

Enhanced X-ray variability from V1647 Ori, the young star in outburst illuminating McNeil's Nebula

We report a ~38 ks X-ray observation of McNeil's Nebula obtained with XMM on 2004 April 4. V1647 Ori, the young star in outburst illuminating McNeil's Nebula, is detected with XMM and appears variable in X-rays. We investigate the hardness ratio variability and time variations of the event energy distribution with quantile analysis, and show that the large increase of the count rate from V1647 Ori observed during the second half of the observation is not associated with any large plasma temperature variations as for typical X-ray flares from young low-mass stars. X-ray spectral fitting shows that the bulk (~75%) of the intrinsic X-ray emission in the 0.5-8 keV energy band comes from a soft plasma component (0.9 keV) reminiscent of the X-ray spectrum of the classical T Tauri star TW Hya, for which X-ray emission is believed to be generated by an accretion shock onto the photosphere of a low-mass star. The hard plasma component (4.2 keV) contributes ~25% of the total X-ray emission, and can be understood only in the framework of plasma heating sustained by magnetic reconnection events. We find a hydrogen column density of NH=4.1E22 cm-2, which points out a significant excess of hydrogen column density compared to the value derived from optical/IR observations, consistent with the picture of the rise of a wind/jet unveiled from ground optical spectroscopy. The X-ray flux observed with XMM ranges from roughly the flux observed by Chandra on 2004 March 22 (~10 times greater than the pre-outburst X-ray flux) to a value two times greater than that caught by Chandra on 2004 March 7 (~200 times greater than the pre-outburst X-ray flux). We have investigated the possibility that V1647 Ori displays a periodic variation in X-ray brightness as suggested by the combined Chandra+XMM data set (abridged).Comment: 11 pages and 8 Figures. Accepted for publication by Astronomy & Astrophysic