Near-IR Spectral Properties of Young Brown Dwarfs
Mike LiuMark PittsJonathan LeongJoan NajitaAlan Tokunaga
Kelle CruzDavy KirkpatrickAdam BurgasserGreg DoppmannDan JaffeKevin LuhmanKatelyn Allers, IfA-Hawaii
Why should we search for young brown dwarfs?Exploring the low mass end of the IMF
Formation Mechanism: Ejection vs. Isolated Collapse
Disks around the lowest-mass objectsImage Credits: NASA/HST; NASA/JPL-Caltech/T. Pyle
Not So Fast!Oph1622-2405 A & B
Mass (A)MJupMass (B)MJup6530352014170.517.52.515.52.555101910/5
Just the M8s
Determining GravityThe 1.14 micron Na doublet is gravity sensitive, and through the late-M and early-L types is only weakly dependent on SpT.
Is there an age (gravity) spread?Line EWs
Na(1.14)K(1.169)K(1.177)K(1.253)TWA3.87 0.751.73 0.502.47 0.321.60 0.26Young2.96 0.540.98 0.211.78 0.221.15 0.36
Determining SpTEven though the overall shapes of the H-band differ, the slope of the blue end of the H-band is SpT sensitive, yet gravity insensitive.
A Custom Filter for Determining SpTCentral wavelength: 1.45 micronsFilter width = 6%Estimated Sensitivity:H=19.9 mag (10 in 1 hour)W~18.0 mag (10 in 1 hour)Designed for 1.0 subtype uncertainty in SpT determination for 5% photometric accuracy W-band detections.Designed for use in 1-5mm of pwv (Mauna Kea)JWH
ConclusionsWe need more young cluster and young field objects in analysis! (Please?)Na and K lines are better gravity indicators than the shape of the H-band continuumWe dont see an age spread indicated in the alkali line EWs. The 1.45 micron steam absorption feature is very sensitive to SpT.
Project started by Mike, Joan and Alan30x30 I-band Ophiuchus 100,000 objects
Matthew Bate, Liz Lada, Mike Meyer have already talked about the importance of the Brown dwarf regime.IMF in the brown dwarf regime can differ based on cloud initial conditionstest predictive theories of SFAlso important for disks (low temperature, tiny vertical gravities) kevin will discuss and tomorrowAll fine and good, but all of these require being able to determine the physical properties (most importantly mass) of the brown dwarfs themselvesThings are never as simple as they seem.My thesis object wide binary (240 AU sep) Oph1622-2405Lyon DUSTY IsochronesShows the results of 4 recent surveysPredicted masses vary by as much as a factor of 5 (or from planetary masses to barely substellar).Whats the deal? Obviously the discrepancy lies not in the luminosity determination but in temperatures.And the spectral types of the primary range from M7-M9So why is this so hard?How do we derive the teffs and Lums?Observed spectrum to SpT: often a qualitative rather than quantitative processWhat you use as a comparison spectrum matters.SpT-Teff relation also are fairly untested and can vary by ~200 KOr comparison to models (*CLICK*) Secondary component of oph1622-2405 with DUSTY models of the high and low of its temp. estimates. Some flavors of models can get better fits. But plots like this make me wary of using atmospheric models to derive object properties. (*CLICK*)Luminosity is a bit more straight forward. Oph1622 has little reddening, and distance to oph doesnt have a lot of spread. Our goal: To characterize young brown dwarfs this project started by obtaining ~20 moderate resolution (750-1800) near-IR spectra using SpeX. When I arrived at Hawaii I added in another ~20 SpeX and GNIRS spectra. Kelle Cruz provided several young field object spectra and weve started obtaining more. Were closing in on having 100 spectra *with SpTs of M5 to L5*
Here are the M7.5 to M8.5 spectra. (explain blue (Taurus, Oph, ChamI, IC348), green and red) triangular shape, and alkali lines
Why Near-IR: extinction and as we move to lower and lower mass objectsFor this analysis we are only using the subset of our sample with SpTs and reddening determined from optical data. First thing we need to determine for young bds is that theyre actually youngexplain both plots. Na feature gets noticibly deeper with age (higher gravity).Ages 1 Myr, 10 Myr and 1 GyrGood separation between the Na feature for young objects and field dwarfs. Were starting to find several young objects in the field...Has little SpT dependenceIf you have an Na Index < 1.1 youre safely youngIf you look at the gravity sensitive lines a little more carefully: we have enough objects that we can do this for each SpT.spread in HR diagram: optimistic about testing disk properties as function of age. Young M8 cluster luminosities from literatureTest afo alkali line strengths (*point out lines*)Problem: we get the same line strengths for all the young objects.We *can* spectroscopically tell the difference between something which is 1 Myr and 10 Myr. And NONE of our M8s are close to 10 Myr. Age Spread in HR diagram is just not indicated in line EWs.So weve looked at L spread. what about SpT?
H band spectra. Point out differences in shape. TWA at 10 Myr and Taurus at 1 Myr(*click*) show the slope in this region is the same regardless of gravityMake and index out of the slope, we get this. The scatter to a linear fit of these points is less than one subtype and it looks like it extends out to all the way to L5.We can SpT new objects!One thing that came out of this analysisOn ULBCAM/UH 88. Sensitivities such that in about 10 hours of observing time we can cover 1 sq degrees to depths (for a local (150 pc) clouds) equivalent to 3 MJ.Cuts out background contaminants so that we can easily select only the brown dwarfs and have few contaminants. Will save LOTS of spectroscopic followup time.Will arrive at the end of this month with our first W-filter run in May. Should have results out soon!