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Register a new userX-ray emission from young brown dwarfs in the Orion Nebula Cluster
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We use the sensitive X-ray data from the Chandra Orion Ultradeep Project (COUP) to study the X-ray properties of 34 spectroscopically-identified brown dwarfs with near-infrared spectral types between M6 and M9 in the core of the Orion Nebula Cluster. Nine of the 34 objects are clearly detected as X-ray sources. The apparently low detection rate is in many cases related to the substantial extinction of these brown dwarfs; considering only the BDs with $A_V leq 5$ mag, nearly half of the objects (7 out of 16) are detected in X-rays. Our 10-day long X-ray lightcurves of these objects exhibit strong variability, including numerous flares. While one of the objects was only detected during a short flare, a statistical analysis of the lightcurves provides evidence for continuous (`quiescent) emission in addition to flares for all other objects. Of these, the $sim$ M9 brown dwarf COUP 1255 = HC 212 is one of the coolest known objects with a clear detection of quiescent X-ray emission. The X-ray properties (spectra, fractional X-ray luminosities, flare rates) of these young brown dwarfs are similar to those of the low-mass stars in the ONC, and thus there is no evidence for changes in the magnetic activity around the stellar/substellar boundary, which lies at $sim$ M6 for ONC sources. Since the X-ray properties of the young brown dwarfs are also similar to those of M6--M9 field stars, the key to the magnetic activity in very cool objects seems to be the effective temperature, which determines the degree of ionization in the atmosphere.
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Rotational studies at a variety of ages and masses are important for constraining the angular momentum evolution of young stellar objects (YSO). Of particular interest are the very low mass (VLM) stars and brown dwarfs (BDs), because of the significant lack of known rotational periods in that mass range. We provide for the first time information on rotational periods for a large sample of young VLM stars and BDs. This extensive rotational period study in the 1 Myr old Orion Nebula Cluster (ONC) is based on a deep photometric monitoring campaign using the Wide Field Imager (WFI) camera on the ESO/MPG 2.2m telescope on La Silla, Chile. Accurate I-band photometry of 2908 stars was obtained, extending three magnitudes deeper than previous studies in the ONC. We found 487 periodic variables with estimated masses between 0.5 Msun and 0.015 Msun, 124 of which are BD candidates. This is by far the most extensive and complete rotational period data set for young VLM stars and BDs. In addition, 808 objects show non-periodic brightness variations. We study the dependence of the period distribution on mass and variability level and compare this with known higher mass objects in the ONC (Herbst et al. 2002) and with the 2 Myr old cluster NGC 2264 (Lamm et al., 2004). We find that substellar objects rotate on average faster than the VLM stars. Our rotational data also suggest a dependence of the rotational periods on position within the field, which can be explained by a possible age spread in the ONC. In addition, periodic variables with larger peak-to-peak amplitudes rotate on average slower than those with small peak-to-peak amplitude variations, which can possibly be explained by different magnetic field topologies.
The XMM-Newton Extended Survey of the TMC (XEST) is a large program designed to systematically investigate the X-ray properties of young stellar/substellar objects in the TMC. In particular, the area surveyed by 15 XMM-Newton pointings (of which three are archival observations), supplemented with one archival Chandra observation, allows us to study 17 BDs with M spectral types. Half of this sample (9 out of 17 BDs) is detected; 7 BDs are detected here for the first time in X-rays. We observed a flare from one BD. We confirm several previous findings on BD X-ray activity: a log-log relation between X-ray and bolometric luminosity for stars (with L*<10 Lsun) and BDs detected in X-rays; a shallow log-log relation between X-ray fractional luminosity and mass; a log-log relation between X-ray fractional luminosity and effective temperature; a log-log relation between X-ray surface flux and effective temperature. We find no significant log-log correlation between the X-ray fractional luminosity and EW(Halpha). Accreting and nonaccreting BDs have a similar X-ray fractional luminosity. The median X-ray fractional luminosity of nonaccreting BDs is about 4 times lower than the mean saturation value for rapidly rotating low-mass field stars. Our TMC BDs have higher X-ray fractional luminosity than BDs in the Chandra Orion Ultradeep Project. The X-ray fractional luminosity declines from low-mass stars to M-type BDs, and as a sample, the BDs are less efficient X-ray emitters than low-mass stars. We thus conclude that while the BD atmospheres observed here are mostly warm enough to sustain coronal activity, a trend is seen that may indicate its gradual decline due to the drop in photospheric ionization degree (abridged).
The Orion Nebula Cluster and the molecular cloud in its vicinity have been observed with the ACIS-I detector on board the Chandra X-ray Observatory with 23 hours exposure. We detect 1075 X-ray sources: 91% are spatially associated with known stellar members of the cluster, and 7% are newly identified deeply embedded cloud members. This provides the largest X-ray study of a pre-main sequence stellar population. We examine here the X-ray properties of Orion young stars as a function of mass. Results include: (a) the discovery of rapid variability in the O9.5 31 M_o star theta^2A Ori, and several early B stars, inconsistent with the standard model of X-ray production in small wind shocks; (b) support for the hypothesis that intermediate-mass mid-B through A type stars do not themselves produce significant X-ray emission; (c) confirmation that low-mass G- through M-type T Tauri stars exhibit powerful flaring but typically at luminosities considerably below the `saturation level; (d) confirmation that the presence or absence of a circumstellar disk has no discernable effect on X-ray emission; (e) evidence that T Tauri plasma temperatures are often very high with T >= 100 MK, even when luminosities are modest and flaring is not evident; and (f) detection of the largest sample of pre-main sequence very low mass objects showing high flaring levels and a decline in magnetic activity as they evolve into L- and T-type brown dwarfs.
(Abridged) Context: Both X-ray and radio observations offer insight into the high-energy processes of young stellar objects (YSOs). The observed thermal X-ray emission can be accompanied by both thermal and nonthermal radio emission. Due to variability, simultaneous X-ray and radio observations are a priori required, but results have been inconclusive. Aims: We use archival X-ray and radio observations of the Orion Nebula Cluster (ONC) to significantly enlarge the sample size of known YSOs with both X-ray and radio detections. Methods: We study the ONC using multi-epoch non-simultaneous archival Chandra X-ray and NRAO Very Large Array (VLA) single-band radio data. The multiple epochs allow us to reduce the impact of variability by obtaining approximated quiescent fluxes. Results: We find that only a small fraction of the X-ray sources (7%) have radio counterparts, even if 60% of the radio sources have X-ray counterparts. The radio flux density is typically too low to distinguish thermal and nonthermal radio sources. Only a small fraction of the YSOs with detections in both bands are compatible with the empirical Guedel-Benz (GB) relation. Most of the sources not compatible with the GB relation are proplyds, and thus likely thermal sources, but only a fraction of the proplyds is detected in both bands, such that the role of these sources is inconclusive. Conclusions: While the radio sources appear to be globally unrelated to the X-ray sources, the X-ray dataset clearly is much more sensitive than the radio data. We find tentative evidence that known non-thermal radio sources and saturated X-ray sources are indeed close to the empirical relation, even if skewed to higher radio luminosities, as they are expected to be. Most of the sources that are clearly incompatible with the empirical relation are proplyds which could instead plausibly be thermal radio sources.
We present the result of a systematic search for the iron Kalpha fluorescent line at 6.4 keV among 1616 X-ray sources detected by ultra-deep Chandra observations of the Orion Nebula Cluster and the obscured Orion Molecular Cloud 1 population as part of the Chandra Orion Ultra-deep Project (COUP). Seven sources are identified to have an excess emission at 6.4 keV among 127 control sample sources with significant counts in the 6.0-9.0 keV band. These seven sources are young stellar objects (YSOs) characterized by intense flare-like flux variations, thermal spectra, and near-infrared (NIR) counterparts. The observed equivalent widths of the line cannot be attributed to the fluorescence by interstellar or circumstellar matter along the line of sight. The X-ray spectral fits and NIR colors of the 6.4 keV sources show that these sources have X-ray absorption of > 1x10^22 cm^(-2) and NIR excess emission, which is not expected when the fluorescence occurs at the stellar photosphere. We therefore conclude that the iron fluorescent line of YSOs arises from reflection off of circumstellar disks, which are irradiated by the hard X-ray continuum emission of magnetic reconnection flares.
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