Ophiuchi Cloud Research Articles

Observations of 6–200 μm emission of the Ophiuchus cloud LDN 1688★

We examine two positions, ON1 and ON2, within the Ophiuchus cloud LDN 1688 using observations made with the ISOPHOT instrument aboard the ISO satellite. The data include mid-IR spectra (~6-12{\mu}m) and several photometric bands up to 200{\mu}m. The data probe the emission from molecular PAH-type species, transiently-heated Very Small Grains (VSGs), and large classical dust grains. We compare the observations to earlier studies, especially those carried out towards an isolated translucent cloud in Chamaeleon (Paper I). The spectra towards the two LDN 1688 positions are very similar to each other, in spite of position ON1 having a larger column density and probably being subjected to a stronger radiation field. The ratios of the mid-IR features are similar to those found in other diffuse and translucent clouds. Compared to paper I, the 7.7/11.3{\mu}m band ratios are lower, ~2.0, at both LDN 1688 positions. A continuum is detected in the ~10{\mu}m region. This is stronger towards the position ON1 but still lower than on any of the sightlines in Paper I. The far-infrared opacities are higher than for diffuse medium. The value of the position ON2, {\tau}200/N(H) = 3.9 x 10^{-25} cm^2/H, is twice the value found for ON1. The radiation field of LDN 1688 is dominated by the two embedded B type double stars, {\rho} Oph AB and HD 147889, with an additional contribution from the Upper Sco OB association. The strong heating is reflected in the high colour temperature, ~24 K, of the large grain emission. Radiative transfer modelling confirms a high level of the radiation field and points to an increased abundance of PAH grains. However, when the hardening of the radiation field caused by the local B-stars is taken into account, the observations can be fitted with almost no change to the standard dust models. However, all the examined models underestimate the level of the mid-IR continuum.

A SIGNIFICANT POPULATION OF CANDIDATE NEW MEMBERS OF THE ρ OPHIUCHI CLUSTER

We present a general method for identifying the pre-main-sequence population of any star-forming region, unbiased with respect to the presence or absence of disks, in contrast to samples selected primarily via their mid-infrared emission from Spitzer surveys. We have applied this technique to a new, deep, wide-field, near-infrared imaging survey of the Rho Ophiuchi cloud core to search for candidate low mass members. In conjunction with published Spitzer IRAC photometry, and least squares fits of model spectra (COND, DUSTY, NextGen, and blackbody) to the observed spectral energy distributions, we have identified 948 candidate cloud members within our 90% completeness limits of J=20.0, H=20.0, and K_S=18.50. This population represents a factor of ~3 increase in the number of known young stellar objects in the Rho Ophiuchi cloud. A large fraction of the candidate cluster members (81% +/- 3%) exhibit infrared excess emission consistent with the presence of disks, thus strengthening the possibility of their being bona fide cloud members. Spectroscopic follow-up will confirm the nature of individual objects, better constrain their parameters, and allow an initial mass function to be derived.

New constraints on the polarization of anomalous microwave emission in nearby molecular clouds

ABSTRACT Anomalous microwave emission (AME) has been previously studied in two well-known molecular clouds and is thought to be due to electric dipole radiation from small spinning dust grains. It is important to measure the polarization properties of this radiation both for component separation in future cosmic microwave background (CMB) experiments and also to constrain dust models. We have searched for linearly polarized radio emission associated with the ρ Ophiuchi and Perseus molecular clouds using WMAP 7-year data. We found no significant polarization within an aperture of 2° diameter. The upper limits on the fractional polarization of spinning dust in the ρ Ophiuchi cloud are 1.7, 1.6 and 2.6 per cent (at 95 per cent confidence level) at K, Ka and Q bands, respectively. In the Perseus cloud we derived upper limits of 1.4, 1.9 and 4.7 per cent, at K, Ka and Q bands, respectively; these are similar to those found by López-Caraballo et al. If AME at high Galactic latitudes has a similarly low level of polarization, this will simplify component separation for CMB polarization measurements. We can also rule out single domain magnetic dipole radiation as the dominant emission mechanism for the 20–40 GHz. The polarization levels are consistent with spinning dust models.

THE STAR FORMATION RATE AND GAS SURFACE DENSITY RELATION IN THE MILKY WAY: IMPLICATIONS FOR EXTRAGALACTIC STUDIES

We investigate the relation between star formation rate (SFR) and gas surface densities in Galactic star forming regions using a sample of YSOs and massive clumps. Our YSO sample consists of objects located in 20 molecular clouds from the Spitzer cores to disks and Gould's Belt surveys. We estimate the gas surface density (Sigma_gas) from Av maps and YSO SFR surface densities (Sigma_SFR) from the number of YSOs, assuming a mean mass and lifetime. We also divide the clouds into contour levels of Av, counting only the youngest Class I and Flat SED YSOs. For a sample of massive star forming clumps, we derive SFRs from the infrared luminosity and use HCN gas maps to estimate Sigma_gas. We find that Galactic clouds lie above the extragalactic relations (e.g., Kennicutt-Schmidt Law) by factors up to 17. Cloud regions with high Sigma_gas lie above extragalactic relations up to a factor of 54 and overlap with massive clumps. We use 12CO and 13CO gas maps of the Perseus and Ophiuchus clouds to estimate Sigma_gas and compare to Sigma_gas from Av maps. We find that 13CO, underestimates the Av-based mass by factors of 4-5. 12CO may underestimate the total gas mass at Sigma_gas > 200 Msun pc^-2 by > 30%;however, this does not explain the large discrepancy between Galactic and extragalactic relations. We find evidence for a threshold of star formation (Sigma_th) at 129+-14 Msun pc^-2. At Sigma_gas > Sigma_th, the Galactic relation is linear. A possible reason for the difference between Galactic and extragalactic relations is that all the CO-emitting gas, including Sigma_gas below Sigma_th, is measured in extragalactic studies. If the Kennicutt-Schmidt relation (Sigma_SFR Sigma_gas^1.4) and a linear relation between dense gas and star formation is assumed, the fraction of dense star forming gas (f_dense) increases as Sigma_gas^0.4. When Sigma_gas reaches ~300Sigma_th, f_dense is 1. (Abridged)

Results from DROXO

X-rays from very young stars are powerful probes to investigate the mechanisms at work in the very first stages of the star formation and the origin of X-ray emission in very young stars. We present results from a 500 ks long observation of the Rho Ophiuchi cloud with a XMM-Newton large program named DROXO, aiming at studying the X-ray emission of deeply embedded Young Stellar Objects (YSOs). The data acquired during the DROXO program were reduced with SAS software, and filtered in time and energy to improve the signal to noise of detected sources; light curves and spectra were obtained. We detected 111 sources, 61 of them associated with rho Ophiuchi YSOs as identified from infrared observations with ISOCAM. Specifically, we detected 9 out of 11 Class I, 31 out of 48 Class II and 15 out 16 Class III objects. Six objects out of 21 classified Class III candidates are also detected. At the same time we suggest that 15 Class III candidates that remain undetected at log Lx < 28.3 are not related to the cloud population. The global detection rate is ~64%. We have achieved a flux sensitivity of ~5 x 10^{-15} erg s^{-1} cm^{-2}. The Lx to L_bol ratio shows saturation at a value of ~ -3.5 for stars with T_eff <= 5000 K or 0.7 M_sun as observed in the Orion Nebula. The plasma temperatures and the spectrum absorption show a decline with YSO class, with Class I YSOs being hotter and more absorbed than Class II and III YSOs. In one star (GY 266) with infrared counterpart in 2MASS and Spitzer catalogs we have detected a soft excess in the X-ray spectrum which is best fitted by a cold thermal component less absorbed than the main thermal component of the plasma. Such a soft component hints to the presence of plasma heated by shocks due to jets outside the dense circumstellar material.

A METHANE IMAGING SURVEY FOR T DWARF CANDIDATES IN ρ OPHIUCHI

We report the results of the first deep, wide-field, near-infrared methane imaging survey of the Rho Ophiuchi cloud core to search for T dwarfs. Among the 6587 objects detected, 22 were identified as T dwarf candidates. Brown dwarf models indicate that at the age and distance of the Rho Ophiuchi cloud, these T dwarf candidates have masses between 1 and 2 Jupiter masses. If confirmed as genuine T dwarfs, these objects would be the youngest, lowest mass, and lowest gravity free-floating objects ever directly observed. The existence of these candidates suggests that the initial mass function of the Rho Ophiuchi cloud extends well into the regime of planetary mass objects. A large fraction (59% +/- 16%) of our T dwarf candidates appear to be surrounded by circumstellar disks, and thus represent the lowest mass objects yet found to harbor circumstellar disks.

OUTFLOWS IN ρ OPHIUCHI AS SEEN WITH THESPITZERINFRARED ARRAY CAMERA

Using the IRAC images from the Spitzer c2d program, we have made a survey of mid-infrared outflows in the rho Ophiuchi molecular cloud. Extended objects that have prominent emission in IRAC channel 2 (4.5 micron) compared to IRAC channel 1 (3.6 micron) and stand out as green objects in the three-color images (3.6 micron in blue, 4.5 micron in green, 8.0 micron in red) are identified as mid-infrared outflows. As a result, we detected 13 new outflows in the rho Ophiuchi molecular cloud that have not been previously observed in optical or near-infrared. In addition, at the positions of previously observed HH objects or near-infrared emission, we detected 31 mid-infrared outflows, among which seven correspond to previously observed HH objects and 30 to near-infrared emission. Most of the mid-infrared outflows detected in the rho Ophiuchi cloud are concentrated in the L1688 dense core region. In combination with the survey results for Young Stellar Objects (YSOs) and millimeter and sub-millimeter sources, the distribution of mid-infrared outflows in the rho Ophiuchi molecular complex hints a propagation of star formation in the cloud in the direction from the northwest to the southeast as suggested by previous studies of the region.

The nature of the Class I population in Ophiuchus as revealed through gas and dust mapping

<i>Context. <i/>The Ophiuchus clouds, in particular L 1688, are an excellent region to study the embedded phases of star formation, due to the relatively large number of protostars. However, the standard method of finding and characterizing embedded young stellar objects (YSOs) through just their infrared spectral slope does not yield a reliable sample. This may affect the age determinations, often derived from the statistics on the total number of embedded YSOs and pre-main sequence stars within a cloud.<i>Aims. <i/>Our aim is to characterize the structure of protostellar envelopes on an individual basis and to correctly identify the embedded YSO population of L 1688.<i>Methods. <i/>Spectral maps of the HCO<sup>+<sup/> 4-3 and C<sup>18<sup/>O 3-2 lines, using the HARP-B array on the James Clerk Maxwell Telescope and SCUBA 850 <i>μ<i/>m dust maps, are obtained of all sources in the L 1688 region with infrared spectral slopes consistent with, or close to, that of embedded YSOs. Selected 350 <i>μ<i/>m maps obtained with the Caltech Submillimeter Observatory are presented as well. The properties, extent and variation of dense gas, column density and dust up to 1<i>'<i/> (~7500 AU) are probed at 15<i>''<i/> resolution. Using the spatial variation of the gas and dust, together with the intensity of the HCO<sup>+<sup/> 4-3 line, we are able to accurately identify the truly embedded YSOs and determine their properties. <i>Results. <i/>The protostellar envelopes range from 0.05 to 0.5 in mass. The concentration of HCO<sup>+<sup/> emission (~0.5 to 0.9) is generally higher than that of the dust concentration. Combined with absolute intensities, HCO<sup>+<sup/> proves to be a better tracer of protostellar envelopes than dust, which can contain disk and cloud contributions. Our total sample of 45 sources, including all previously classified Class I sources, several flat-spectrum sources and some known disks, was re-classified using the molecular emission. Of these, only 17 sources are definitely embedded YSOs. Four of these embedded YSOs have little (0.1-0.2 ) envelope material remaining and are likely at the interesting transitional stage from embedded YSO to T Tauri star. About half of the flat-spectrum sources are found to be embedded YSOs and about half are disks.<i>Conclusions. <i/>The presented classification method is successful in separating embedded YSOs from edge-on disks and confused sources. The total embedded population of the Ophiuchus L 1688 cloud is found almost exclusively in Oph-A, Oph-B2 and the Ophiuchus ridge with only three embedded YSOs not related to these regions. The detailed characterization presented will be necessary to interpret deep interferometric ALMA and future Herschel observations.

The initial conditions of star formation - VIII. An observational study of the Ophiuchus cloud L1688 and implications for the pre-stellar core mass function

We reanalyse all of the archived observations of the Ophiuchus dark cloud L1688 that were carried out with the submillimetre common-user bolometer array on the James Clerk Maxwell Telescope. For the first time, we put together all the data that were taken of this cloud at different times to make a deeper map at 850 μm than has ever been published. Using this new, deeper map, we extract the pre-stellar cores from the data. We use updated values for the distance to the cloud complex, and also for the internal temperatures of the pre-stellar cores to generate an updated core mass function (CMF). This updated CMF is consistent with previous results in so far as they went, but our deeper map gives an improved completeness limit of 0.1 M⊙ (0.16 Jy), which enables us to show that a turnover exists in the low-mass regime of the CMF. The L1688 CMF shows the same form as the stellar initial mass function (IMF), and can be mapped on to the stellar IMF, showing that the IMF is determined at the pre-stellar core stage. We compare L1688 with the Orion star-forming region and find that the turnover in the L1688 CMF occurs at a mass roughly a factor of 2 lower than the CMF turnover in Orion. This suggests that the position of the CMF turnover may be a function of environment.

THE KINEMATICS OF MOLECULAR CLOUD CORES IN THE PRESENCE OF DRIVEN AND DECAYING TURBULENCE: COMPARISONS WITH OBSERVATIONS

In this study we investigate the formation and properties of prestellar and protostellar cores using hydrodynamic, self-gravitating Adaptive Mesh Refinement simulations, comparing the cases where turbulence is continually driven and where it is allowed to decay. We model observations of these cores in the C$^{18}$O$(2\to 1)$, NH$_3(1,1)$, and N$_2$H$^+(1\to 0)$ lines, and from the simulated observations we measure the linewidths of individual cores, the linewidths of the surrounding gas, and the motions of the cores relative to one another. Some of these distributions are significantly different in the driven and decaying runs, making them potential diagnostics for determining whether the turbulence in observed star-forming clouds is driven or decaying. Comparing our simulations with observed cores in the Perseus and $\rho$ Ophiuchus clouds shows reasonably good agreement between the observed and simulated core-to-core velocity dispersions for both the driven and decaying cases. However, we find that the linewidths through protostellar cores in both simulations are too large compared to the observations. The disagreement is noticably worse for the decaying simulation, in which cores show highly supersonic infall signatures in their centers that decrease toward their edges, a pattern not seen in the observed regions. This result gives some support to the use of driven turbulence for modeling regions of star formation, but reaching a firm conclusion on the relative merits of driven or decaying turbulence will require more complete data on a larger sample of clouds as well as simulations that include magnetic fields, outflows, and thermal feedback from the protostars.

A Study of the ρ Ophiuchi Cloud: Mapping the Distribution and the Motions of Interstellar Gas

Guest Investigator results from Hipparcos, combined with high-resolution optical spectra obtained with the Anglo-Australian Observatory, Kitt Peak National Observatory, and McDonald Observatory, are used to create a three-dimensional map of the stars and several interstellar gas components in the ? Oph cloud complex. Many principal gas components are interspersed among the bright stars, which can be subdivided into three main groupings on the basis of right ascension and declination. At least one of these main gas components may persist over about 10 pc laterally, and there is evidence for an overall bulk movement of the gas, as determined from the observed radial velocities. We find the diffuse portion of the cloud complex to be clumpy, scattered over a distance range of ~110-150 pc. We find the most likely distance to the dense molecular cloud to be -->122 ? 8 pc.

Signatures of Dynamical Star Formation in the Ophiuchus Association of Pre–Main‐Sequence Stars

A sample of 58 probable members of the association of pre-main-sequence stars around the filamentary ρ Ophiuchi cloud is investigated. Using astrometric proper motions from the UCAC-2 catalog and the convergent point method, the total heliocentric systemic velocity is estimated at 19 km s^(-1), and the mean distance at 145 pc. A small, statistically insignificant difference between the geometric convergent point and the actual direction of velocity defined by the observed radial velocity implies a small, if any, rate of expansion of the association. The Ophiuchus association appears to have a considerable depth, with half of the members lying within 15 pc of the center. When the moving cluster distances are taken into account, the near-infrared M_(Ks) versus (J - K_s) H-R diagram reveals an apparent large spread of ages between 14 Myr and younger than 1 Myr. Most of the youngest stars are located along a slightly curved strip just south of the densest cloud, extending in the east-west direction roughly aligned with the central streamer of warm dust. The intersection of this strip with a thin segment of shocked dust visible in the IRAS 12 μm map at (α,δ) = (16^h31^m, −24.5°) is marked with a small cluster of probably very young stars around ROXs 43A. The large extent and depth of the association, the moderate rate of expansion, the spread in ages of about 14 Myr, and the alignment of very young stars with the dusty streamer point at a dynamical mode of star formation in this region, scattered far and wide around the main core.

Spatial separation of small and large grains in the transitional disk around the young star IRS 48

Aims. We present spatially resolved mid-infrared images of the disk surrounding the young star IRS 48 in the Ophiuchus cloud complex. The disk exhibits a ring-like structure at 18.7 mu m, and is dominated by very strong emission from polycyclic aromatic hydrocarbons at shorter wavelengths. This allows a detailed study of the relative distributions of small and large dust grains. Methods. Images of IRS 48 in 5 mid-infrared bands from 8.6 to 18.7 mu m as well as a low resolution N-band spectrum are obtained with VLT-VISIR. Optical spectroscopy is used to determine the spectral type of the central star and to measure the strength of the H alpha line. Results. The 18.7 mu m ring peaks at a diameter of 110 AU, with a gap of similar to 60 AU. The shape of the ring is consistent with an inclination of i = 48 degrees +/- 8 degrees. In contrast, the 7.5 - 13 mu m PAH emission bands are centered on the source and appear to fill the gap within the ring. The measured PAH line strengths are 10 - 100 x stronger than those typically measured for young M0 stars and can only be explained with a high PAH abundance and/or strong excess optical/UV emission. The morphology of the images, combined with the absence of a silicate emission feature, imply that the inner disk has been cleared of micron-sized dust but with a significant population of PAHs remaining. We argue that the gap can be due to grain growth and settling or to clearing by an unseen planetary or low-mass companion. IRS 48 may represent a short-lived transitional phase from a classical to a weak-line T Tauri star.

A CO ( J = 3-2) Outflow Survey of the Elias 29 Region

We present a 5' × 5' integrated intensity map of 12CO (J = 3-2) emission from the ρ Ophiuchi cloud core that traces low-luminosity outflow emission from two protostars: Elias 29 and, most likely, LFAM 26. The morphology of the outflow from Elias 29 is bipolar and has a curved axis that traces the S-shaped symmetry seen in H2 emission. The outflow from LFAM 26 is a new detection and oriented in the east-west direction near the plane of the sky with most of the blueshifted emission being absorbed by intervening clouds. The outflow axis of this object also appears to intersect a knot of H2 emission previously attributed to Elias 29. LFAM 26 is a low-luminosity source (Lbol = 0.06 L☉), which, in combination with the observed outflow, makes it a candidate very low luminosity object (VeLLO). We derive lower limits to the gas column densities and energetics for both outflows. The mechanical luminosities for Elias 29 and LFAM 26 are 6.4 and 10.3 × 10-3 L☉, respectively.

Radial and rotational velocities of young brown dwarfs and very low-mass stars in the Upper Scorpius OB association and the Ophiuchi cloud core

We present the results of a radial velocity (RV) survey of 14 brown dwarfs (BDs) and very low-mass (VLM) stars in the Upper Scorpius OB association (UScoOB) and three BD candidates in the ρ Ophiuchi dark cloud core. We obtained high-resolution echelle spectra at the Very Large Telescope using the Ultraviolet and Visual Echelle Spectrograph (UVES) at two different epochs for each object, and measured the shifts in their RVs to identify candidates for binary/multiple systems in the sample. The average time separation of the RV measurements is 21.6 d, and our survey is sensitive to the binaries with separation<0.1 au. We found that four out of 17 objects (or 24+16 −13 per cent by fraction) show a significant RV change in 4–33 d time-scale, and are considered as binary/multiple ‘candidates’. We found no double-lined spectroscopic binaries in our sample, based on the shape of cross-correlation curves. The RV dispersion of the objects in UScoOB is found to be very similar to that of the BD and VLM stars in Chamaeleon I (Cha I). We also found the distribution of the mean rotational velocities (v sin i) of the UScoOB objects is similar to that of the Cha I, but the dispersion of v sin i is much larger than that of the Cha I objects.

First Evidence of a Precessing Jet Excavating a Protostellar Envelope

We present new, sensitive, near-infrared images of the Class I protostar, Elias 29, in the Ophiuchus cloud core. To explore the relationship between the infall envelope and the outflow, narrowband H2 1-0 S(1), Br-gamma, and narrowband K-continuum filters were used to image the source with the Wide-Field Infrared Camera on the Hale 5m telescope and with Persson's Auxiliary Nasmyth Infrared Camera on the Baade 6.5 m telescope. The source appears as a bipolar, scattered light nebula, with a wide opening angle in all filters, as is typical for late-stage protostars. However, the pure H2 emission-line images point to the presence of a heretofore undetected precessing jet. It is argued that high-velocity, narrow, precessing jets provide the mechanism for creating the observed wide-angled outflow cavity in this source.

Submillimetre observations of low‐mass cloud cores: forming tiny objects in situ

AbstractThe origin of very low‐mass objects such as brown dwarfs and ‘isolated planets’ is unclear: can they form in‐situ from very low‐mass cloud cores in a scaled‐down version of star formation? Here I discuss methods of detecting and characterising such faint cores using submillimetre‐wavelength observations. Some data are presented for the Ophiuchus clouds that strongly suggest there is little division between stars and ultra low‐mass objects at the earliest evolutionary stages. Some challenging results have emerged (in the context of current theory), including finding cores of only a few Jupiter masses and a core mass function still rising at the mass detection limit: the implications are briefly discussed. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

IRAS 16293−2422: Proper Motions, Jet Precession, the Hot Core, and the Unambiguous Detection of Infall

We present high spatial resolution observations of the multiple protostellar system IRAS 16293-2422 using the Submillimeter Array (SMA) at 300 GHz and the Very Large Array (VLA) at frequencies from 1.5 to 43 GHz. This source was already known to be a binary system with its main components, A and B, separated by ~5''. The new SMA data now separate source A into two submillimeter continuum components, which we denote Aa and Ab. The strongest of these, Aa, peaks between the centimeter radio sources A1 and A2, but the resolution of the current submillimeter data is insufficient to distinguish whether this is a separate source or the centroid of submillimeter dust emission associated with A1 and A2. Archival VLA data spanning 18 yr show proper motion of sources A and B of 17 mas yr-1, associated with the motion of the ? Ophiuchi cloud. We also find, however, significant relative motion between the centimeter sources A1 and A2, which excludes the possibility that these two sources are gravitationally bound unless A1 is in a highly eccentric orbit and is observed at periastron, the probability of which is low. A2 remains stationary relative to source B, and we identify it as the protostar that drives the large-scale northeast-southwest CO outflow. A1 is shock-ionized gas that traces the location of the interaction between a precessing jet and nearby dense gas. This jet probably drives the large-scale east-west outflow, and indeed its motion is consistent with the wide opening angle of this flow. The origin of this jet must be located close to A2 and may be the submillimeter continuum source Aa. Thus, source A is now shown to comprise three (proto)stellar components within 1''. Source B, on the other hand, is single, exhibits optically thick dust emission even at 8 GHz, has a high luminosity, and yet shows no sign of outflow. It is probably very young and may not even have begun a phase of mass loss yet. The SMA spectrum of IRAS 16293-2422 reports the first astronomical identification of many lines of organic and other molecules at 300 and 310 GHz. The species detected are typical of hot cores, the emission from which is mainly associated with source A. The abundances of second-generation species, especially of sulphur-bearing molecules, are significantly higher than predicted by chemical models for this source to date, and we suggest that shocks are probably needed to explain these enhancements. The peaks in the integrated emission from molecules having high rotation temperatures coincide with the centimeter source A1, also highlighting the key role of shocks in explaining the nature of hot cores. Finally, we use the high brightness temperature of the submillimeter dust emission from source B to demonstrate the unambiguous detection of infall by observing redshifted SO (77-66) absorption against the emission from its dust disk.

The X-ray emission from Young Stellar Objects in the ρ 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.

A Mid‐Infrared Imaging Survey of Embedded Young Stellar Objects in the ρ Ophiuchi Cloud Core

Results of a comprehensive, new, ground-based mid-infrared imaging survey of the young stellar population of the (rho) Ophiuchi cloud are presented. Data were acquired at the Palomar 5m and at the Keck 10m telescopes with the MIRLIN and LWS instruments, at 0'.5 and 0'.25 resolutions, respectively. Of 172 survey objects, 85 were detected. Among the 22 multiple systems observed, 15 were resolved and their individual component fluxes determined. A plot of the frequency distribution of the detected objects with SED spectral slope shows that YSOs spend approx.4 x 10(exp 5) yr in the flat-spectrum phase, clearing out their remnant infall envelopes. Mid-infrared variability is found among a significant fraction of the surveyed objects and is found to occur for all SED classes with optically thick disks. Large-amplitude near-infrared variability, also found for all SED classes with optically thick disks, seems to occur with somewhat higher frequency at the earlier evolutionary stages. Although a general trend of mid-infrared excess and near-infrared veiling exists progressing through SED classes, with Class I objects generally exhibiting r(sub K) >= 1, flat-spectrum objects with r(sub K) >= 0.58, and Class III objects with r(sub K) =0, Class II objects exhibit the widest range of r(sub K) values, ranging from 0 <= r(sub K) <= 4.5. However, the highly variable value of veiling that a single source can exhibit in any of the SED classes in which active disk accretion can take place is striking and is direct observational evidence for highly time-variable accretion activity in disks. Finally, by comparing mid-infrared versus near-infrared excesses in a subsample with well-determined effective temperatures and extinction values, disk-clearing mechanisms are explored. The results are consistent with disk clearing proceeding from the inside out.