Searching for Substellar Objects

Michael McElwain

Advisor: Dr. James Larkin

Science in collaboration with Dr. Adam Burgasser

Overview

Introduction to Brown Dwarfs Lick Wide-Field T Dwarf Search

2 field T dwarf discoveries First substellar subdwarf discovered

Digital Filtering to Search for Substellar Companions in the Halo of Nearby Stars Advantage of using OSIRIS data cubes Quicklook v2.0

Brown Dwarf Formation & Evolution Brown dwarfs have masses below the

hydrogen burning minimum mass (HBMM)

log(age) Gyrs log(age) Gyrs

Teff Klog(L) Ls

Burrows et al. 1997

Searches for Brown Dwarfs

Companions to nearby stars Direct Imaging Infrared speckle imaging Coronographic imaging Radial velocities Adaptive Optics Imaging

Young clusters Field Searches

*2MASS

T dwarfs

Spectral features dominated by CH4, H2O, CIA H2, and K I absorption features.

T < ~1350 K MJ ~ 14-16

T dwarf Near Infrared Spectra

Why Search for Additional Field T Dwarfs?

Only 39 T dwarfs known Improved search parameters Larger samples needed to lower uncertainties in

substellar properties substellar statistics in the Solar Neighborhood substellar mass function

Discover unique objects Discover cooler substellar objects Characterize substellar properties

Sample Coverage & Selection Techniques

Sample Selection d > -20° 15° < |b| < 88° J < 16 J-H < 0.3 H-Ks < 0 No optical counterpart within

5” of the 2MASS coordinates on the USNO-A2.0 catalog

2MASS database is flagged to ignore detections of known minor planets

L

M

T

Selection Techniques

267,646 candidates pass the initial selection criterion.

Visual examination of DSS images for faint optical counterparts to candidates

Roughly 99.5% of candidates are removed in the visual examination

Reimaging Campaign

Gemini Infrared Camera (Lick Observatory) Uncataloged minor planets remain

Reimage field at J & K to confirm the presence of a candidate Roughly 25% of the remaining candidates are removed in this

manner

Palomar 60” CCD Camera Identify faint background stars

r-J < 6 that can pass initial criteria Roughly 80% of the remaining candidates are removed in this

manner

Spectroscopy

Gemini Infrared Camera Simultaneous observations at J/K or H/K Low resolution (/ 500)

CH4 + H2O absorption detection a decisive test for identifying T dwarfs

Comparison spectra were taken for known M, L, and T dwarfs

J Comparison Spectra

T dwarfs are recognized by the strong CH4 absorption and increased H2O absorption.

M3V

M8V

L1V

sdL

M6V

T2V

T5.5V

Results

13 spectra of candidates 2MASS 0516-0445 (mid T)

J-K=-0.5 2MASS 1503+2525 (T 5.5V)

J=13.9, third brightest T dwarf known

D ~ 8pc 2MASS 0532+8246 SdL

First Substellar Subdwarf Strong CIA H2 absorption in K

band

2MASS 0532+8246 SdL

2MASS 0516-0445 (mid T)

2MASS 1503+2525 (T 5.5V)

Substellar Companions Multiple systems occur in roughly 60% of solar-

type systems (Duquennoy & Mayor 1991)

Rate of multiplicity decreases for lower mass stars “Brown Dwarf Desert” to solar type stars reported

by radial velocity measurements a < 4AU (Marcy & Benitez 1989)

~35% of field M dwarfs, a ~ 3-30AU (Fischer & Marcy 1992, Henry & McCarthy 1993, Reid & Gizis 1997)

~20% of field L dwarfs, a < 15 AU (Koerner et al. 1999, Reid et al. 2001, Leggett et al. 2001, Close et al. 2002)

~20% of field T dwarfs, a < 3AU (Burgasser et al. 2003)

Using OSIRIS to Search for Substellar Companions

High angular resolution Keck telescope Keck AO system

Moderate Spectral Resolution Obtain simultaneous

spatial and spectral information

If unresolved, use spectral information to search for companions

Digital Filtering of OSIRIS Data Cubes

Many reasons to apply digital filters to OSIRIS data cubes Suppress OH contamination Make a K´ or Ks image from the broadband K Simulate JHK filter transmissions from other

instruments or telescopes Search for substellar companions in the halos

of nearby stars

Weighted Digital Filter for Substellar Companions When looking for a companion in

the halo of the host star, the spectra will constructively interfere within a particular spatial element in the OSIRIS data cube.

L1V

G8V

(L1V/ G8V)-1

Quicklook v2.0

Comprehensive 3 dimensional image analysis tool written in IDL conforming to Keck coding standards

Object Oriented Program, capable of managing multiple windows

Typical image manipulation functions as well as more specific image analysis tools

Quicklook v2.0 Plots

Ability to take cuts of the data cube in multiple orientations.

Easily customized plot parameters

Can set QL2 to remember plot parameters.

Applying Digital Filters with QL2

QL v2 is supplied with a tool to apply digital filters.

2 column data containing wavelength and multiplication factor are read.

Data is sampled onto the OSIRIS wavelength grid and displayed in a plot window.

Can apply and remove filters from the image in the image window gui.

Simulation of the Keck AO Point Spread Function

Wavelength coverage 2.0-2.3 µm and a sampling rate of 0.002 µm.

ro of 0.3 at 0.5 µm Strehl ratio of 0.6 Plate scale of 0.02 "/pix 2 second integration time

Bruce MacIntosh, LLNLProblem!

Digital Filter Applied to the Host and Target Simulated with the Simulated PSF

Simulated PSF produces an inaccurate representation of the halo for a 15 minute exposure

Resolved components increased contrast by a factor of 10.

Before filter After filter

Conclusions

Lick Wide Field T Dwarf Search identified two T dwarfs, one substellar subdwarf

OSIRIS is an ideal instrument to search for substellar companions Simultaneous spatial and spectral information Advantage of using OSIRIS data cubes

Quicklook v2.0 is a comprehensive 3 dimensional analysis software