Methane T-Dwarf Candidates in the Star Forming Region IC 348
aa r X i v : . [ a s t r o - ph ] O c t Methane T-Dwarf Candidates in the StarForming Region IC 348
A. Burgess, J. Bouvier and E. Moraux
Laboratoire d’Astrophysique, Observatoire de Grenoble, Université J. Fourier, CNRS, BP 53,38041 Grenoble Cedex 9, France
Abstract.
IC 348 is a young (t ∼ ∼ on/off) wide-field cameras on the Canada-France-HawaiiTelescope. From the analysis of the narrowband CH on/off deep images, we report 4 T-dwarfcandidates, of which 3 clearly lie within the limits of the IC 348 cluster. An upper limit on theextinction was estimated for each candidate from colour-magnitude diagrams, and found consistentwith extinction maps of the cloud. Initial comparisons with T-dwarf spectral models suggest thesecandidates have a spectral type between T3 and T5, and perhaps later, potentially making theseamong the lowest mass isolated objects detected in a young star forming region so far. Keywords:
Stars: formation; Stars: low-mass, brown dwarfs; Galaxy: open clusters and associa-tions: IC348
PACS:
INTRODUCTION
In the framework of the EU Marie Curie network “Constellation : the origin of stellarmasses”, we conducted a deep imaging survey of the star forming region IC 348 [5] withthe aim to find isolated planetary mass objects, with a mass of a few M
Jup , and constrainthe low mass end of the Initial Mass Function. Deep, wide-field broad-band zJHK andnarrow-band CH4 on/off images were obtained at CFHT. Image analysis focused onfaint source detection, and a number of photometric tests were conducted to ensure goodphotometric reliability and completeness down to K ∼ SELECTION OF METHANE CANDIDATES CH off (1.58 µ m) and CH on (1.69 µ m) narrowband filters were used to detectmethane absorption bands that develop in the atmosphere of the coolest (T ≤ on-CH off colours as a function of spectral type. The results areshown in Figure 1, with the L/T dwarf transition occurring at CH on-CH off ≃ off image and PSF photometry wasapplied to both the CH on and CH off images. In order to conservatively account for the IGURE 1.
Left :
Spectra of field T-dwarfs overlain by the WIRCAM CH on/off filter transmissioncurves (dashed line). The CH on (1.69 µ m) filter is centered on the methane absorption band while theCH off (1.58 µ m) filter measures the nearby pseudo-continuum. T-dwarfs with deep methane absorptionare expected to have larger CH on-CH off colours than non-methane dwarfs. Right :
CFHT CH on-CH off synthetic colours versus spectral type for field L1-T8 dwarfs. The L/T transition occurs at CH on-CH off ∼ photometric error, we selected T-dwarf candidates as having CH on–CH off ≥ on-CH offversus CH off diagram (Fig. 2), along with all the other stellar-like objects detected inthe images. Cand-3589 remained undetected on the CH on image and has been plottedusing the CH on detection limit ( ≃ σ or more above the rms photometric error (0.12 mag), as measured by the dispersionof the background population in the same magnitude bin. PROPERTIES OF CANDIDATE T-DWARFS
An estimate of the extinction towards each candidate is required to derive their absolutemagnitude and, using model isochrones, mass. An upper limit on the extinction wasobtained from the J versus (J-H) diagram shown in Figure 2 by projecting the candidatesback onto the COND isochrone. The results are summarised in Table 1 and concurwith extinction maps of IC 348 by [4] and [6] which indicate 4 ≤ A V ≤
20 mag forcloud members. Table 1 also lists the candidates magnitude and their spectral typeestimated from Figure 2. Note, however, that the spectral classification is uncertain, asthe comparison between IC 348 methane dwarfs and much older field T-dwarfs in Fig. 2assumes that their widely different gravity does not impact on their methane colours.
IGURE 2.
Left :
Observed CH on-CH off colours versus CH off magnitude for stars in our survey.The 4 candidate T-dwarfs are shown as blue squares. The field T0-T7 dwarf sequence, shifted to a distanceof 340 pc, is shown for reference (red triangles). At an age of 3 Myr, IC 348 T-dwarfs are expected to bebrighter than field dwarfs, as are the 4 identified candidates. An extinction vector of A V =10 mag is shown.Note the asymmetric distribution of the CH on-CH off colours in this diagram, with more field objectswith negative values, due to extinction. Right :
J versus (J-H) colour-magnitude diagram. Dusty (black)and Cond (magenta) 3 Myr isochrones are shown as solid lines labelled with mass (M ⊙ ). An A V =10 magextinction vector is shown. IC 348 T-dwarf candidates are shown as blue squares. They lie between Dustyand Cond isochrones as expected for intermediate T-dwarfs. CLOUD MEMBERSHIP
The probability of one of the candidates being a foreground field T-dwarf projectedagainst the cloud instead of being a bona fide IC 348 member can be estimated from theexpected number density of T3-5.5 dwarfs in the solar neighourhood, ∼ − [7]. The footprint of the CH image is ∼ , which at the distance of IC 348( ∼ ′ of the cluster centre (cf. Table 1), the half-mass radius of which is 5 ′ . We concludethat at least 3 of the 4 proposed T-dwarf candidates are probable IC 348 members, witha mass of a few jupiter masses according to theoretical models at an age of ∼ CONCLUSION
We report the detection of 4 methane dwarf candidates in the young star forming regionIC 348. We tentatively estimate a spectral type in the range T3-T5 or even later, whichwould make these candidates amongst the least massive isolated objects ever detected in
ABLE 1.
Photometry, estimated extinction (upper limit) and spectral type for the 4 T-dwarfcandidates. The distance of each candidate from the cluster centre (03 h m s ; +32 ◦ ′ ′′ ,J2000) is listed in the last column. When the candidate remained undetected in one filter, themagnitude listed is the detection limit, indicated with a ’ ∗ ’.Object z J H K CH off CH on A V /mag Sp.T. Dist.-CH off868 23.83 21.92 21.12 19.9 20.19 0.49 6.4 T3 13.4 ′ ′ ≥ ∗ ′ ≥ ∗ ≥ T6 1.5 ′ a star forming region. Although follow up observations are clearly needed, spectroscopicconfirmation will be difficult owing to the faintness of the candidates. ACKNOWLEDGMENTS
This research is supported by the Marie Curie Research Training Network "CONSTEL-LATION" under grant no. MRTN-CT-2006-035890 . This research has made use of theNASA/ IPAC Infrared Science Archive, which is operated by the Jet Propulsion Labo-ratory, California Institute of Technology, under contract with the National Aeronauticsand Space Administration. This research has also made use of the SIMBAD database,operated at CDS, Strasbourg, France. Many thanks also goes to P. Delorme for comput-ing the empirical sequence of L1-T8 field dwarfs used in the figures of this paper. REFERENCES
1. F. Allard, P. H. Hauschildt, D. R. Alexander, A. Tamanai, and A. Schweitzer, ApJ , 357–372 (2001)2. G. Chabrier, I. Baraffe, F. Allard, and P. Hauschildt, ApJ , 464–472 (2000)3. I. Baraffe, G. Chabrier, T. S. Barman, F. Allard, and P. H. Hauschildt, A&A , 701–712 (2003)4. K. Cernis,
Baltic Astronomy , 214–+ (1993).5. A. A. Muench, C. J. Lada, K. L. Luhman, J. Muzerolle, and E. Young, AJ , 411–444 (2007)6. A. A. Muench, E. A. Lada, C. J. Lada, R. J. Elston, J. F. Alves, M. Horrobin, T. H. Huard, J. L. Levine,S. N. Raines, and C. Román-Zúñiga, AJ , 2029–2049 (2003)7. S. A. Metchev, J. D. Kirkpatrick, G. B. Berriman, and D. Looper, ApJ , 1281–1306 (2008)1