Ophiuchi Cluster Research Articles

A Radial Velocity Survey of Embedded Sources in the Rho Ophiuchi Cluster

Abstract We present the results of a radial velocity survey of young stellar objects (YSOs) in early stages of evolution in the core of the L1688 molecular cloud. New and archival spectra obtained with four high-resolution infrared spectrographs were analyzed using Markov chain Monte Carlo techniques that simultaneously fit for the radial velocity, T eff, v sin i, and veiling by comparison with synthetic spectra. The radial velocity distribution for 32 objects, most with Class I or flat-spectrum spectral energy distributions, is marginally Gaussian, with a higher dispersion relative to optical surveys at the 2σ level. When comparing the results from both proper-motion and radial velocity surveys in L1688, there is a trend for the 1D dispersions to be higher for samples of Class I/flat-spectrum YSOs that reside in the cloud core compared to Class II/III dominated samples, which are located in the lower extinction periphery. In addition, there is a velocity gradient along the major axis of the cloud core that appears more pronounced than that derived from optically visible objects at the cloud edges. If these higher dispersions for Class I/flat-spectrum objects are confirmed by future surveys, this could imply a supervirial state for the less evolved objects in the cloud core and be a signature of the initial collapse and rebound of the cluster as suggested by recent simulations of cluster evolution.

Herschelsurvey of brown dwarf disks inρOphiuchi

Recent observations of the Rho Ophiuchi cluster with the Herschel Space Observatory allow us to probe the spectral energy distribution (SED) of the brown dwarf population in the far-IR, where the disk emission peaks. We performed aperture photometry at 70, 100, and 160 micron, and constructed SEDs for all previously known brown dwarfs detected. These were complemented with ancillary photometry at shorter wavelengths. We compared the observed SEDs to a grid of synthetic disks produced with the radiative transfer code MCFOST, and used the relative figure of merit estimated from the Bayesian inference of each disk parameter to analyse the structural properties. We detected 12 Class II brown dwarfs with Herschel, which corresponds to one-third of all currently known brown dwarf members of Rho Ophiuchi. We do not detect any of the known Class III brown dwarfs. Comparison to models reveals that the disks are best described by an inner radius between 0.01 and 0.07 AU, and a flared disk geometry with a flaring index between 1.05 and 1.2. Furthermore, we can exclude values of the disk scale-height lower than 10 AU (measured at a fiducial radius of 100 AU). We combined the Herschel data with recent ALMA observations of the brown dwarf GY92 204 (ISO-Oph 102), and by comparing its SED to the same grid of disk models, we derived an inner disk radius of 0.035 AU, a scale height of 15 AU with a flaring index of beta~1.15, an exponent for dust settling of -1.5, and a disk mass of 0.001 MSun. This corresponds to a disk-to-central object mass ratio of ~1%. The structural parameters constrained by the extended SED coverage (inner radius and flaring index) show a narrow distribution for the 11 young brown dwarfs detected in Rho Ophiuchi, suggesting that these objects share the same disk evolution and, perhaps, formation.

SUBSTELLAR OBJECTS IN NEARBY YOUNG CLUSTERS (SONYC). V. NEW BROWN DWARFS IN ρ OPHIUCHI

SONYC - Substellar Objects in Nearby Young Clusters - is a survey program to investigate the frequency and properties of substellar objects with masses down to a few times that of Jupiter in nearby star-forming regions. For the ~1Myr old rho Ophiuchi cluster, in our earlier paper we reported deep, wide-field optical and near-infrared imaging using Subaru, combined with 2MASS and Spitzer photometry, as well as follow-up spectroscopy confirming three likely cluster members, including a new brown dwarf with a mass close to the deuterium-burning limit. Here we present the results of extensive new spectroscopy targeting a total of ~100 candidates in rho Oph, with FMOS at the Subaru Telescope and SINFONI at the ESO's Very Large Telescope. We identify 19 objects with effective temperatures at or below 3200 K, 8 of which are newly identified very-low-mass probable members of rho Oph. Among these eight, six objects have Teff <= 3000 K, confirming their likely substellar nature. These six new brown dwarfs comprise one fifth of the known substellar population in \rho Oph. We estimate that the number of missing substellar objects in our survey area is ~15, down to 0.003 - 0.03 MSun and for Av = 0 - 15. The upper limit on the low-mass star to brown dwarf ratio in rho Oph is 5.1 +- 1.4, while the disk fractions are ~40% and ~60% for stars and BDs, respectively. Both results are in line with those for other nearby star forming regions.

Strong accretion on a deuterium-burning brown dwarf

Context. The accretion processes that accompany the earliest stages of star formation have been shown in recent years to extend to masses well below the substellar limit, and even to masses close to the deuterium-burning limit, suggesting that the features characteristic of the T Tauri phase are also common to brown dwarfs. Aims. We discuss new observations of GY 11, a young brown dwarf in the embedded ρ Ophiuchi cluster. Methods. We have obtained for the first time low-resolution, long-slit spectroscopy of GY 11 in the red visible region, using the FORS1 instrument at the VLT. The spectral region includes accretion diagnostic lines such as Hα and the CaII infrared triplet. Results. The visible spectrum allows us to confirm that GY 11 lies well below the hydrogen-burning limit, in agreement with earlier findings based on the near-infrared spectral energy distribution. We obtain an improved derivation of its physical parameters, which suggest that GY 11 is on or near the deuterium-burning phase. We estimate a mass of 30 MJup, a luminosity of 6 × 10 −3 L� ,a nd a temperature of 2700 K. We detect strong Hα and CaII triplet emission, and estimate from the latter an accretion rate u Macc =

The dynamics of an extremely young cluster probed for the first time

AbstractWe present a proper-motion program that we have developed at ESO to measure, for the first time, proper motions of members of the nearby (125 pc), very young (4 × 105 yr) infrared ρ Ophiuchi cluster. Repeated imaging over an extended period will allow us to measure the global proper motion of the cluster and its velocity dispersion and will provide access to the ~ 1 km s−1 velocities for objects as faint as K = 15.7 mag. Access to the velocity field of such an extremely young cluster is of crucial importance to investigate the origin and early evolution of open cluster systems.

Deep near-IR variability survey of pre-main-sequence stars in ρ Ophiuchi

Context. Variability is a common characteristic of pre-main-sequence stars (PMS). Near-IR variability surveys of young stellar objects (YSOs) can probe stellar and circumstellar environments and provide information about the dynamics of the ongoing magnetic and accretion processes. Furthermore, variability can be used as a tool to uncover new cluster members in star formation regions. Aims. We hope to achieve the deepest near-IR variability study of YSOs targeting the ρ Ophiuchi cluster. Methods. Fourteen epochs of observations were obtained with the Wide Field Camera (WFCAM) at the UKIRT telescope scheduled in a manner that allowed the study of variability on timescales of days, months, and years. Statistical tools, such as the multi-band cross correlation index and the reduced chi-square, were used to disentangle signals of variability from noise. Variability characteristics are compared to existing models of YSOs in order to relate them to physical processes, and then used to select new candidate members of this star-forming region. Results. Variability in the near-IR is found to be present in 41% of the known population of ρ Ophiuchi recovered in our sample. The behaviours shown are several and can be associated with the existence of spots on the stellar surface, variations in circumstellar extinction, or changes in the geometry of an accretion disc. Using variability, a new population of objects has been uncovered that is believed to be part of the ρ Ophiuchi cluster.

Disc signatures in a new population of low mass YSOs in ρ Ophiuchi

AbstractThe deepest near-IR variability survey of the ρ Ophiuchi cluster with the WFCAM/UKIRT has been used to uncover a new population of young low mass objects. Evidence for the existence of discs around the new objects has been found when combining data with IRAC/Spitzer observations. A new insight on the frequency and dynamics of discs around low mass YSOs is given, essential for understanding the origin and evolution of circumstellar discs and ultimately the environment for planet formation.

Spectroscopy of a Young Brown Dwarf in the ρ Ophiuchi Cluster

We present observations of a young (~3-10 Myr old) brown dwarf within the ρ Ophiuchi star-forming region. A low-resolution optical spectrum exhibits a spectral type of M8.5 ± 0.5 and strong Hα emission (Wλ ~ 60 A). After demonstrating the youth of this source through several additional lines of evidence, we place it on the H-R diagram and use the latest evolutionary tracks to derive a substellar mass of 0.01-0.06 M☉. A more accurate mass estimate will require tests of the evolutionary tracks and temperature scales at such young ages and late types.

Properties of Low‐Mass Objects in NGC 2024

We have obtained deep JHK imaging of the central region of the NGC 2024 embedded cluster, allow- ing three-color measurements of 151 sources. We show that contamination by background sources is small, and we reject a small number of sources that are probably foreground. We use a Monte Carlo isochrone -tting technique to estimate the luminosities and masses of the low-mass members of the embedded cluster. We -nd that the IMF between D0.04 and D0.5 can be -tted within the errors M _ with a power law of slope (1.2 in linear mass units (or (0.2 in logarithmic units). There is no indica- tion of a turnover of the IMF at the bottom of the main sequence; it appears to continue down to D0.04 with no signi-cant deviation (within our moderately large errors) from the power law. This M _ estimate of the IMF includes only the primary members of binary (or multiple) systems. Of a subset of 35 sources, 24 (69%) appear to have infrared excesses indicating possible circumstellar disks, supporting arguments that disk formation is not inhibited in high-density clusters. Subject headings: H II regions E ISM: individual (NGC 2024) E open clusters and associations: individual (NGC 2024) E stars: low-mass, brown dwarfs E stars: luminosity function, mass function and (3) the large number of sources allows the embedded cluster to be studied with good statistics. In this paper, we present the results of a new, deep near- infrared survey of NGC 2024 in the J, H, and K bands, which samples most of the area covered by the embedded cluster. The infrared photometry of the sources is analyzed by -tting models of young stars and substellar objects to determine the mass function. Our analysis is based on a technique developed in a previous study of the o Ophiuchi cluster et al. hereafter and

The Low-Mass IMF in the rho Ophiuchi Cluster

The Low-Mass IMF in the rho Ophiuchi Cluster

The Stellar Population in the rho Ophiuchi Cluster

We report a near-infrared survey of 200 arcmin 2 in the cores of the ρ Ophiuchi interstellar clouds, leading to the detection of 91 sources. We develop a new method for estimating the luminosities of these sources based on fitting of isochrones from theoretical models. Application of this method allows us to determine a luminosity function and accompanying initial mass function for the cluster of young stars forming in these clouds. The IMF extends to well below the bottom of the main sequence (0.08 M ○. ) with no obvious feature at the transition between stellar and substellar masses. The power-law exponent of the IMF at low stellar and substellar masses is roughly −1.3 to −1.1, with formal errors of 0.2

A study of the Chamaeleon I dark cloud and T-association – V. Luminosity function for members⋆

We have compiled a list of 81 probable members in the Chamaeleon I star forming region. New ground-based infrared photometry is presented and infrared spectral energy distributions (SEDs) are constructed by combining ground-based and IRAS data. Classification based on the spectral slope of the SEDs between 2.2 and 25-mu-m indicates that Chamaeleon I is on the average in a later stage of evolution than the rho Ophiuchi cluster. Having extended the SEDs to shorter wavelengths by including results from optical photometry published in the literature, we deduced bolometric luminosities for 62 objects. The resulting luminosity function (LF) of Chamaeleon I is very similar to that of Taurus. We cannot detect the segregation between embedded and visible objects observed in the LF of the rho Ophiuchi cluster. In order to gain a qualitative understanding of this difference, we propose a scenario in which the most massive subregions of clumpy molecular clouds start producing stars before the smaller ones.