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Register a new userSpectroscopy of planetary mass brown dwarfs in Orion
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P. W. Lucas
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We report the results of near infrared spectroscopy of 11 luminosity selected candidate planetary mass objects (PMOs) in the Trapezium Cluster with Gemini South/GNIRS and Gemini North/NIRI. 6 have spectral types >=M9, in agreement with expectations for PMOs. 2 have slightly earlier types, and 3 are much earlier types which are probably field stars. 4/6 sources with types >= M9 have pseudo-continuum profiles which confirm them as low gravity cluster members. The gravity status of the other cool dwarfs is less clear but these remain candidate PMOs. The derived number fraction of PMOs with M=3-15 Mjup is 1-14%, these broad limits reflecting the uncertainty in source ages. However, the number fraction with M<20 Mjup is at least 5%. These detections add significantly to the body of evidence that a planetary mass population is produced by the star formation process.
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We present the results of a H- and K-band multi-object and long-slit spectroscopic survey of substellar mass candidates in the outer regions of the Orion Nebula Cluster. The spectra were obtained using MOIRCS on the 8.2-m Subaru telescope and ISLE on the 1.88-m telescope of Okayama Astronomical Observatory. Eight out of twelve spectra show strong water absorptions and we confirm that their effective temperatures are < 3000 K (spectral type > M6) from a chi-square fit to synthetic spectra. We plot our sources on an HR diagram overlaid with theoretical isochrones of low-mass objects and identify three new young brown dwarf candidates. One of the three new candidates is a cool object near the brown dwarf and planetary mass boundary. Based on our observations and those of previous studies, we determine the stellar (0.08 < M/Msun < 1) to substellar (0.03 < M/Msun < 0.08) mass number ratio in the outer regions of the Orion nebular cluster to be 3.5 +/- 0.8. In combination with the number ratio reported for the central region (3.3+0.8/-0.7), this result suggests the number ratio does not simply change with the distance from the center of the Orion nebular cluster.
We present near-infrared long slit and multi-slit spectra of low mass brown dwarf candidates in the Orion Nebula Cluster. The long slit data were observed in the H- & K-bands using NIRI on the Gemini North Telescope. The multi-object spectroscopic observations were made using IRIS2 on the Anglo Australian Telescope at H-band. We develop a spectral typing scheme based on optically calibrated, near infrared spectra of young sources in the Taurus and IC 348 star forming regions with spectral types M3.0 to M9.5. We apply our spectral typing scheme to 52 sources, including previously published UKIRT and GNIRS spectra. 40 objects show strong water absorption with spectral types of M3 to >M9.5. The latest type objects are provisionally classified as early L types. We plot our sources on H-R diagrams overlaid with theoretical pre-main-sequence isochrones. The majority of our objects lie close to or above the 1 Myr isochrone, leading to an average cluster age that is <1 Myr. We find 38 sources lie at or below the hydrogen burning limit (0.075 Msun). 10 sources potentially have masses below the deuterium burning limit (0.012 Msun). We use a Monte Carlo approach to model the observed luminosity function with a variety of cluster age and mass distributions. The lowest chi^2 values are produced by an age distribution centred at 1 Myr, with a mass function that declines at sub-stellar masses according to an M^alpha power law in the range alpha=0.3 to 0.6. We find that truncating the mass function at 0.012 Msun produces luminosity functions that are starved of the faintest magnitudes, even when using bimodal age populations that contain 10 Myr old sources. The results of these Monte Carlo simulations therefore support the existence of a planetary mass population in the ONC.
Measurement of the substellar initial mass function (IMF) in very young clusters is hampered by the possibility of the age spread of cluster members. This is particularly serious for candidate planetary mass objects (PMOs), which have a very similar location to older and more massive brown dwarfs on the Hertzsprung-Russell Diagram (HRD). This degeneracy can be lifted by the measurement of gravity-sensitive spectral features. To this end we have obtained medium resolution (R~5000) Near-infrared Integral Field Spectrometer (NIFS) K band spectra of a sample of late M- / early L-type dwarfs. The sample comprises old field dwarfs and very young brown dwarfs in the Taurus association and in the Sigma Orionis cluster. We demonstrate a positive correlation between the strengths of the 2.21micron NaI doublet and the objects ages. We demonstrate a further correlation between these objects ages and the shape of their K band spectra. We have quantified this correlation in the form of a new index, the H2(K) index. This index appears to be more gravity-sensitive than the NaI doublet and has the advantage that it can be computed for spectra where gravity-sensitive spectral lines are unresolved, while it is also more sensitive to surface gravity at very young ages (<10 Myr) than the triangular H band peak. Both correlations differentiate young objects from field dwarfs, while the H2(K) index can distinguish, at least statistically, populations of ~1 Myr objects from populations of ~10 Myr objects. We applied the H2(K) index to NIFS data for one Orion nebula cluster (ONC) PMO and to previously published low resolution spectra for several other ONC PMOs where the 2.21micron NaI doublet was unresolved and concluded that the average age of the PMOs is ~1 Myr.
We report the results of a deep near infrared (JHK) survey of the outer parts of the Trapezium Cluster with Gemini South/Flamingos. 396 sources were detected in a 26 arcmin^2 area, including 138 brown dwarf candidates, defined as M<0.075 Msun for an assumed age of 1 Myr. Only 33 of the brown dwarf candidates are planetary mass candidates (PMCs) with estimated masses in the range 0.003<M<0.012Msun. In an extinction limited sample (A(V)<5) complete to approximately 0.005 Msun (5 Mjup) the mass function appears to drop by a factor of 2 at the deuterium burning threshold, i.e. at planetary masses. After allowing for background contamination it is likely that planetary mass objects at 3-13 Mjup number <10% of the cluster population, with an upper limit of 13%. Analysis of the spatial distribution of stars and brown dwarf candidates suggests that brown dwarfs and very low mass stars (M<0.1 Msun) are less likely than more massive stars to have wide (>150 AU) binary companions. This result has modest statistical significance (96%) in our data but is supported at 93% confidence by analysis of an completely independent sample taken from the Subaru data of Kaifu et al.(2000). There is a statistically very significant excess of both stars and brown dwarfs with small separations from each other (<6 arcsec or 2600 AU). This appears to be due to the presence of small N subgroups, which are likely to be dynamically unstable in the long term. Hence these results are consistent with the ejected stellar embryo hypothesis for brown dwarf formation (Reipurth & Clarke 2001). We also report the discovery of two new bipolar nebulae, which are interpreted as Class I protostars.
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.
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