Infrared Spectroscopy of Substellar Objects in Orion

By P. Lucas, P. Roche, F. Allard, Peter Hauschildt. 2001

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We present broad band spectra of a sample of 21 low luminosity sources in the Trapezium Cluster, with masses in the range 0.008 - 0.10 M⊙ (assuming an age of 1 Myr). These were selected for low extinction in most cases and are located west of the brighter nebulosity. The spectra are in the H bandpass (1.4-1.95 µm) and K bandpass (1.9-2.5 µm) also for most of the brighter sources, with a resolution of 50 nm. They were taken with the United Kingdom Infrared Telescope (UKIRT) using the CGS4 spectrometer. Absorption by water vapour bands is detected in all the substellar candidates except one, which is a highly reddened object with strong H2 emission and an anomalously blue (I-J) colour, implying that it is a very young cluster member with circumstellar matter. The observation of prominent water vapour bands confirms the low Effective Temperatures implied by our (I-J) colour measurements in an earlier paper and would imply late M or L spectral types if these were older field dwarfs. However, the profiles of the H bandpass spectra are very different from those of field dwarfs with similar water absorption strength, demonstrating that they are not foreground or background objects. In addition, the CO absorption bands at 2.3 µm and the NaI absorption feature at 2.21 µm are very weak for such cool sources. All these features are quite well reproduced by the AMES-Dusty-1999 model atmospheres of Allard et al.(2000,2001), and arise from the much lower gravities predicted for the Trapezium sources (3.5 < log(g) < 4.0) compared to evolved objects (log g � 5.5), This represents a new proof of the substellar status of our sources, independent of the statistical arguments for low contamination, which are reexamined here. The very late spectral types of the planetary mass objects and very low mass brown dwarfs demonstrate that they are cluster members, since they are too luminous to be field dwarfs in the background. We also present additional UKIRT photometry of a small region in the south of the Trapezium cluster where the extinction and nebular brightness are low, which permitted the detection of objects with 1 Myr masses slightly lower than our previous least massive source at 8 MJup. Following a minor update to our previous J band photometry, due to a new UKIRT filter calibration, there are � 15 planetary mass candidates in the full dataset.

Subfields: Astronomy and Astrophysics, Atmospheric Science, Spectroscopy

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Citations in the corpus

A Census of the Young Cluster IC 348 (2003, background): “The offset in classifications in Figure 7 is probably due to differences in the steam band strengths between dwarfs and young objects at a given spectral type, which is a behavior that has been noticed previously (Luhman &Rieke 1999; Lucas et al. 2001; Allard et al. 2001).”
HST SPECTRAL MAPPING OF L/T TRANSITION BROWN DWARFS REVEALS CLOUD THICKNESS VARIATIONS (2013, background): “…underluminosity has also been reported for a handful of field brown dwarfs (e.g. HD 203030B: Metchev & Hillenbrand 2006, HN Peg B: Luhman et al. 2007, 2MASSJ18212815+1414010, 2MASSJ21481628+4003593: Looper et al. 2008) and young brown dwarfs in clusters (Lucas et al. 2001; Allers et al. 2006).”
Chandra Orion Ultradeep Project: Observations and Source Lists (2004, background): “Spectral types in Column 7 are from H97 updated with the work of Luhman et al. (2000) and Lucas et al. (2001).”
THE HAWAII INFRARED PARALLAX PROGRAM. II. YOUNG ULTRACOOL FIELD DWARFS (2016, background): “(2) Young objects can be distinguished from older objects as their lower surface gravities are manifested as differences in their optical and near-infrared (IR) spectra compared to field objects (e.g. Mart́ın et al. 1999b; Lucas et al. 2001).”
The Stellar Population of the Chamaeleon I Star-forming Region (2007, background): “Similarly, the triangular H-band continuum of Cha J11083040-7731387 is indicative of youth (Lucas et al. 2001), but this object appears to have strong K I absorption at 1.25 µm, which suggests higher, dwarf-like gravity.”
YOUNG L DWARFS IDENTIFIED IN THE FIELD: A PRELIMINARY LOW-GRAVITY, OPTICAL SPECTRAL SEQUENCE FROM L0 TO L5 (2008, background): “Near-infrared (1–2.5 µm) spectra of the faintest young cluster brown dwarfs are more feasible to obtain and have revealed candidate L dwarf members of Orion, Chameleon II, Ophiuchus, and Upper Scorpius (Lucas et al. 2001; McGovern et al. 2004; Allers et al. 2007; Lodieu et al. 2008).”
CLOUDS AND CHEMISTRY IN THE ATMOSPHERE OF EXTRASOLAR PLANET HR8799b (2011, background): “The shape of the H band is often used as a gravity indicator in L-type brown dwarfs, where low gravity objects have distinct triangular spectra (Lucas et al. 2001; Kirkpatrick et al. 2006) compared to the more rounded spectra of older, higher gravity, field brown dwarfs.”
Discovery of a Very Young Field L Dwarf, 2MASS J01415823–4633574 (2005, background): “Further evidence of youth is found in the near-infrared spectrum, including a triangular-shaped H-band continuum reminiscent of young brown dwarf candidates discovered in the Orion Nebula Cluster.”
The Disk Population of the Chamaeleon I Star-forming Region (2008, background): “Finally, the spectral features that are sensitive to surface gravity in our data, such as the shape of the H- and K-band continua (Lucas et al. 2001; Luhman et al. 2004; Kirkpatrick et al. 2006), are inconsistent with field dwarfs and giants.”
BANYAN. VII. A NEW POPULATION OF YOUNG SUBSTELLAR CANDIDATE MEMBERS OF NEARBY MOVING GROUPS FROM THE BASS SURVEY (2015, background): “…plateau at 2.18–2.28µm (see the H2(K) index of Canty et al. 2013), leaving the effect of water vapor to become apparent from the triangular-shaped continuum of the H band (Lucas et al. 2001; Kirkpatrick et al. 2006; Allers et al. 2007; Rice et al. 2010; see the H-cont index of Allers & Liu 2013).”
The formation of a star cluster: predicting the properties of stars and brown dwarfs (2002, background): “…to probe masses down to this theoretical minimum mass (Zapatero Osorio et al. 1999; Lucas & Roche 2000; Béjar et al. 2001; Mart́in et al. 2001b; Lucas et al. 2001), with the masses of some objects estimated to be as low as 3 Jupiter-masses (Zapatero Osorio et al. 2002a; McCaughrean 2003).”
Inverse dynamical population synthesis: Constraining the initial conditions of young stellar clusters by studying their binary populations (2012, unknown): “Brown dwarfs have since been detected in the centre of the cluster (Hillenbrand & Carpenter 2000; Luhman et al. 2000; Lucas et al. 2001; Muench et al. 2002), so that the mean mass may decreases to 0.5M⊙ (Scally et al. 2005), in good agreement with the canonical IMF.”
Discovery of Two Nearby Peculiar L Dwarfs from the 2MASS Proper-Motion Survey: Young or Metal-Rich? (2008, background): “Reduced CIA H2 and/or increased condensate opacity are likely responsible for the triangular-shaped H-band spectra characteristic of low-gravity, late-type M and earlytype L dwarfs seen both in young clusters (e.g., Orion: Lucas et al. 2001; Cha II, Rho Oph: Allers et al. 2007) and associations (e.g”
The Formation and Early Evolution of Low-Mass Stars and Brown Dwarfs (2012, background): “Figure 3 also shows how the shape of the H-band continuum varies with gravity, exhibiting a broad plateau in old dwarfs and a triangular peak in young objects (Lucas et al. 2001).”
Evolutionary models for low-mass stars and brown dwarfs: uncertainties and limits at very young ages (2001, background): “Although still very preliminary, the compari- son of observed and synthetic spectra, as recently done by Lucas et al. (2001) for Orion objects, provides a promising way to define such a Teff - scale.”
Galactic Stellar and Substellar Initial Mass Function (2003, background): “…limit and below the deuterium-burning minimum mass, and the rising mass spectrum down to this limit in several young clusters (see Figure 5 and Najita et al. 2000, Béjar et al. 2001, Mart́ın et al 2001, Lucas et al. 2001) seem to exclude deuterium burning as a peculiar process in star formation.”
THE EXTREMELY RED, YOUNG L DWARF PSO J318.5338−22.8603: A FREE-FLOATING PLANETARY-MASS ANALOG TO DIRECTLY IMAGED YOUNG GAS-GIANT PLANETS (2013, background): “5−22 shows a triangularH-band continuum, which is considered a hallmark of youth (e.g. Lucas et al. 2001).”
The Initial Mass Function in the Taurus Star-forming Region (2002, background): “…dwarfs have concentrated on star-forming clusters like the Orion Nebula cluster (Hillenbrand 1997; Luhman et al. 2000; Hillenbrand & Carpenter 2000; Lucas et al. 2001; Muench et al. 2002), Orionis (Barrado y Navascués et al. 2001; Béjar et al. 2001), IC 348 (Herbig 1998; Luhman 1999), and…”