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METHANE IMAGING SURVEY FOR PLANETARY MASS OBJECTS IN RHO OPHIUCHI
Karl Haisch Jr.1; Mary Barsony2; Chris Tinney3
1Utah Valley University, 2SFSU, 3Univ. of New South Wales
ABSTRACT
T dwarfs are the coolest and least massive compact astrophysical objects that we can directly observe outside our Solar System. They
share many properties with the expanding population of known exoplanets (almost all of which are inaccessible to direct observation
themselves). An understanding of T dwarf atmospheres, therefore, is critical to our developing understanding of exoplanets. Moreover,
T dwarf surveys in young star clusters can provide important answers to questions such as “Is there a minimum mass for star
formation?” and “How important is dynamical evolution for cluster mass functions?”. In recent years, methane imaging has emerged
as a powerful tool for identifying T dwarf candidates in very young clusters, where T dwarfs are at their brightest and have not yet
been subject to possible dynamical ejection from their clusters. We have recently conducted a methane imaging survey of the Rho
Ophiuchi cloud for T dwarfs using the IRIS2 near-infrared imaging spectrograph on the Anglo-Australian Telescope. The Rho
Ophiuchi cloud is the closest (d ~ 120 parsecs) star-forming region to Earth that has a compact core harboring several hundred young
(~1 Myr) stellar objects. In this poster, we present the results of our imaging survey.
Fig. 2.— IRIS2 methane filters. The thick dashed lines show the measured bandpasses of the IRIS2
CH4s and CH4l filters. The cut-on, central, and cutoff wavelengths for the CH4s and CH4l filters are
1.520, 1.570, and 1.620 µm and 1.640, 1.690, and 1.740 µm, respectively. For comparison, the
MKO H filter bandpass (also installed in IRIS2) is plotted as a dotted line. Spectra of the T5 dwarf
2MASS J15031961+2525196 (lower solid line; SpeX data from Burgasser et al. 2004) and the L7
dwarf DENIS-P J0205.4-1159 (upper solid line; CGS4 data from Leggett et al. 2001) are also
shown. The two spectra have been normalized at 1.57 µm. (Fig 1., C. G. Tinney, et al. 2005)
J, Ks, and CH4 Observations
•  IRIS2 on the Anglo-Australian 4.0 meter telescope.
•  IRIS2 plate scale 0.45 arcsec/pixel.
•  Filters used: IRIS2 J (1.245 µm), Ks (2.144 µm); CH4s (1.570 µm), CH4l (1.690 µm).
•  Total on-source integration time at each position was 5 minutes for the J and Ks filters,
and 16 minutes for the CH4s and CH4l filters.
FIG. 3.—IRIS2 methane color (CH4s – CH4l) as a function of J - H, H - K, and J - K (MKO) color.
The CH4s – CH4l uncertainties plotted are the combination of photon-counting uncertainties,
aperture correction uncertainties, and photometric calibration uncertainties. JHK photometry and
uncertainties are from the UKIRT World Wide Web pages, MKO photometry is from Leggett et al.
(2002), and 2MASS photometry is converted to the MKO system using the conversion relations of
Stephens & Leggett (2004). Note the four white dwarf observations that, when plotted in this plane,
lie just to the left of the A0 end of the plotted sequence. (Fig 3., C. G. Tinney, et al. 2005)
Fig. 4.— Variation in broadband H apparent magnitude with age and mass for substellar objects
at a distance of 300 pc; from Burrows et al. (2001). (Fig 2., A. K. Mainzer, et al. 2003) Using the
data acquired from our search we will be able to distinguish objects to 2MJ.
Results
•  We have obtained near-infrared J, Ks, CH4s, & CH4l observations of a 35 x 35 square
arc-minute region centered on the Rho Ophiuchi cloud core.
•  We find a total of 90 T dwarf candidates out of ~6000 total stars examined.
•  Follow-up spectroscopy of candidate T dwarfs found in this survey will be done to
confirm their status as very low mass objects. We will also obtain methane imaging of
at least four further star forming regions.
References
Mainzer, A. K. & McLean, I. S. 2003, AJ, 597, 555
FIG. 1.— Near-infrared JHK image of the Rho Ophiuchi cluster (Robert Hurt, Spitzer Science Center). The numbered
grid overlay corresponds to the regions surveyed. The distance to core of the Rho Ophiuchi region is ~120 pc.
(Loinard et al. 2008).
Tinney, C. G., Burgasser, A. J., Kirkpatrick, J. D., & McEwain, M. W., 2005, AJ, 130, 2326
Loinard, L., Torres, R. M., Mioduszewski, A. J., & Rodriguez, L. F., 2008, AJ, 675 L29