A New Method to Identify Nearby, Young, Low-mass Stars - David Rodriguez

A New Method to Identify Nearby, Young, Low-mass Stars

Author: David Rodriguez

The Main Point: Young, low mass stars (K5 and later, 10-100 Myr) tend to have more UV emission than older stars. So we can use GALEX near-UV (NUV) fluxes to identify a sample of young, nearby stars.

Background:

Why do we care about low mass stars?

• find low mass stars in nearby moving groups with known ages
• use these stars to study the evolution of stars and planetary systems

Why use UV emission?

• young low mass stars are bright in X ray (Zuckerman & Song 2004)
• X-ray emission traces coronal activity, which is correlated with chromospheric activity (Mamajek & Hillenbrand 2008)
• chromospheric activity is associated with higher UV emission (Guinan & Engle 2009)

Why use GALEX?

• GALEX All-Sky Imaging Survey (AIS) covered 3/4 of the sky
• doesn't point within 10 degrees of galactic plane
• PSF = 5", cannot resolve small separation binary systems
• 3-sigma sensitivity for 100 seconds = 22 magnitudes in NUV
• stars should be detected out to ~150 pc
• pointing uncertainty ~1" (Morressey et al. 2007)

Method

The Sample

• Start with stars from Torres et al. 2008, which identifies probable members of nearby moving groups - this is the "young stars" sample
• add in stars from the "Catalog of Nearby Stars, 3rd Edition" - this is the "old stars" sample
• Query the GALEX database using CasJobs

Caveats

• reddening can affect the placement on a color-color diagram
• reddening curves from Cardelli et al. 1989 --> A(NUV)/A(V)=2.96
• extinction towards younger, distant objects will be higher
• the effect is to bring the young and old stars closer on a color-color diagram

Results

• GALEX vs. Vmags 
• young stars (Torres) stand out against older stars (Stauffer)

• GALEX vs. 2MASS
• galaxies lie between 0.5<J-K<2 and 2<NUV-J<7
• stars follow a line between (0,2)<(J-K,NUV-J)<(1,13)
• Here, the young stars are TWA members 

Proper Motions and UVWs

• PM Catalogs:
• USNO-B1
• UCAC3
• PPMX
• PPMXL
• Radial Velocity catalogs:
• DENIS
• when RV's were not available, used a range of reasonable RV's to estimate UVWs
• 
  
• Distances
• photometric, using isochrones (10-100Myr)
• UVWs
• "good box" has UVWs within 0 to -15, -10 to -34, and +3 to -20. Stars in this box are likely younger than 100 Myr (Zuckerman & Song 2004)

Spectroscopy

• stars which appear young according to their UVWs are tagged for spectroscopic follow-up
• presence of Li absorption feature (6708 A) is a strong signature of youth
• TiO bands can be used for spectral typing (Reid et al. 1995)
• H-alpha can help identify T Tauri Stars
• Spectrometers used:
• Wide Field Spectrograph at Siding Springs Observatory (R=3000 or 7000 depending on settings)
• noteworthy targets were followed up with R=7000 spectra.

Conclusions

• One may use excess UV emission to identify young, low-mass stars
• of 24 stars assumed to be young form their UV emission, 17 show strong Li absorption