Simon et al. 2012 Report

This paper, called "WISE Detection of the Circumstellar Disk Associated with 2MASS ______ in the Eta Cha Cluster" actually began with a search for nearby young moving groups (NYMGs) observed with the WISE all-sky survey. The authors wanted to search for circumstellar disks (which they use interchangably with "debris disks") around these young stars (young meaning <100 Myr). Unfortunately, they found very few disks around these young stars in the four WISE bands. WISE observed about 64% of their targets. Of course, this only considers the preliminary data release (about half the sky), so it is likely that all of their targets were observed with WISE. They observed targets in several clusters of varying ages - from eta cha at 6 Myr to AB Dor at 70 Myr. They found that disk incidence declined (from about 35% in Eta Cha to 0 in AB Dor) with cluster age. According to their findings, debris disks are not observed by WISE around stars >100 Myrs old.

This presents a potential problem for my research. If only stars <100 Myrs can be expected to have IR excesses indicative of a debris disk, my method of age determination needs to be changed. Gyrochronology and chromospheric activity is not a valid age tracer for such young stars. Lithium depletion, however may prove to be useful. While this is not a very precise measurement for age, it can tell us about the relative age (plots of EW(Li) vs. L_IR/L_bol might be useful).

That being said, several papers have found debris disks around older stars. Fujiwara et al. 2012 reports a debris disk around HD 15407A (age somewhere between 80 Myr and 2 Gyr). Urban et al. 2012 reports several debris disks in the Hyades cluster (~600-700 Myr). Carpenter et al. 2009 uses Spitzer data to analyze the evolution of debris disks with time. They find that most stars with debris disks are <300 Myr old. It is thought that older stars do have debris disks, but that they were too dim to be reliably detected with IRAS, Spitzer, or Herschel. WISE data may reveal most debris disks around older stars.

Summary: Rhee et al. 2007

Characterization of Dusty Debris Disks: The IRAS and HIPPARCOS Catalog

Purpose: cross-correlate Hipparcos stars with IRAS

Introduction

• definition of a debris disk: excess flux (w.r.t. the model stellar photosphere) betweeen 25-100 um
• >10 Myrs old --> the dust is second generation
• recall LHB ~ within 600 Myr
• IRAS and SPITZER searches
• as of 2007, several hundred IR-excess stars
• estimate dust mass from submm flux and/or tau
• tau=Lir/Lbol
• predict mass from tau and radius
• THE PROBLEM: most IR excess stars are isolated field stars with poorly determined ages. If we want to choose stars with IR excesses in stellar groups, we are getting a limited sampling of ages. 

Search and Selection Technique

• distance limited to 120 pc
• 60 um IRAS sources
• SED fitting using chi^2 minimization
• stellar photosphere from Hauschildt et al. 1999
• requires fluxes at B, V, J, H, and Ks 
• convert Tycho B and V from table 2 in Bessell (200)
• (or from simbad)
• JHKs came from 1MASS
• convert mags to flux densities (janskies) using Cox 2000
• use a chi^2 fitting with stellar radius and effective temperature as free parameters
• "convolve the filter function with the model spectra" = ???