Finding Ultracool Brown Dwarfs with UKIDSS +

SDSS + IRTF/SpeX

Kuenley Chiu, Andy Bunker (Exeter/AAO)

Michael Liu, Katelyn Allers, Trent Dupuy (IfA/Hawaii)

Linhua Jiang, Xiaohui Fan (Arizona)

Dan Stark (Caltech), Karl Glazebrook (Swinburne)

MNRAS, in press

Why hunt for T dwarfs (and why with IRTF/SpeX)?

• T dwarfs are the coldest, faintest objects outside the Solar System that can be directly imaged.

• Teff < 1300 K, Lbol < 10-5 Lsun

• Probes of “stellar” physics in the ultracool regime.• Key pathway for studying extrasolar gas giant planets.

• Most of emergent flux is in the near-IR, and thus key diagnostics of their properties (T, g, Z) are at these wavelenths.

• IRTF/SpeX is especially well suited for these studies.

The newly operational UKIRT Infrared Deep Sky Survey (UKIDSS) has been generating approx. 250 square degrees of YJHK imaging data per semester, overlapping the SDSS, and provides the first significant new dataset to explore T dwarfs since 2MASS.

Using optical dropout techniques, we have been selecting and following up faint red point source candidates, seeking late T dwarfs (and Y dwarfs) and high-redshift quasars.

0.8 sq deg tiled footprint

SDSS

High-z quasars and T dwarfs display similar strong color decrements between the optical (i & z) and the near-IR (Y & J).

Based on spectra of template objects, color selections are carried out on the matched SDSS optical and UKIDSS near-IR catalogs to identify candidate objects.

JJ J

H K

i z Y J H

model z=7 quasar

known T8

Finding optical dropouts with SDSS + UKIDSS

Optical and near-IR confirmation imaging identifies genuine promising objects, and photometry + colors discriminate between BDs and quasars.

Optical spectroscopy is used to identify high-z QSOs (Ly- around 9000A), and IRTF/Spex near-IR spectroscopy used to classify brown dwarfs.

SpeX prism mode offers excellent sensitivity and gets 0.8-2.5 m in one shot.

We classified T dwarfs as faint as J=18.8 (Vega), probably the faintest IR spectra ever published from a 3-4m class telescope.

Four new T dwarfs found, the coolest one being T7.5 (three plotted here).

Chiu et al 2008, MNRAS

SpeX prism mode offers excellent sensitivity and gets 0.8-2.5 m in one shot.

We classified T dwarfs as faint as J=18.8 (Vega), probably the faintest IR spectra ever published from a 3-4m class telescope.

Four new T dwarfs found, the coolest one being T7.5 (three plotted here).

Chiu et al 2008, MNRAS

The 4 T dwarfs found are among the faintest yet known, and found via optical dropout, complementary to IR-only selection.

The sample continues to grow, ~2 dozen T dwarfs now known from UKIDSS data, providing rich followup observations.

Goal is to get the space densities of these very low mass objects.

Chiu et al 2008, MNRAS