Mid-Infrared Followup of Cold Brown Dwarfs: Diversity in Age, Mass and Metallicity

1. Mid-Infrared Followup of Cold Brown Dwarfs: Diversity in Age, Mass and Metallicity Sandy Leggett (Gemini), Ben Burningham and David Pinfield (Hertfordshire UK), Mark Marley (NASA Ames), Didier Saumon (Los Alamos) and Steve Warren (Imperial College London) Gemini NIRI AO image of HR 8799 bcd; Marois et al. 2008 Science 322. Brown Dwarfs and Exoplanets Shanghai China July 2009

2. The T Dwarf Population • The number of published T dwarfs is 155 (e.g. DwarfArchives.org) - less than half the number of exoplanets (353, e.g. exoplanet.eu). • There are 26 published T7 - T9 dwarfs with Teff 900 - 600K. Currently there are three T9s with Teff 500 - 600K known (Warren et al. 2007, Burningham et al. 2008, Delorme et al. 2008). • The known >T7 dwarfs were discovered by the near-IR surveys 2MASS, UKIDSS and CFHT BDS (e.g. earlier talks by Pinfield, Delorme) and in one case as a companion to a planet-host star in near-and mid-IR imaging data. Brown Dwarfs and Exoplanets Shanghai China July 2009

3. The Importance of the Mid-IR with Decreasing Temperature • The fraction of the total flux emitted at >3m increases rapidly for late T dwarfs; for Teff<700K more than 50% of the flux is emitted in the mid-IR. • The mid-IR missions Spitzer and WISE are crucial. The warm Spitzer mission retains imaging capability at 3.6m and 4.5m. Brown Dwarfs and Exoplanets Shanghai China July 2009

4. T8-T9 Spectral Energy Distributions Spitzer IRS spectra show the dominance of the mid-IR as Teff drops from 750K to 600K. The flux emerges between strong bands of H2O and CH4. NH3 is likely to be present in the near-IR also, for T9 dwarfs. The Spitzer IRAC and WISE bands sample regions of low and high flux, allowing the detection of cool objects and their identification by their unusual (extreme) colors. Brown Dwarfs and Exoplanets Shanghai China July 2009

5. Colors and Type Near IR: The cloud decks impact late-L to early-T. The blue trends for the Ts, due to increasing CH4 and H2O absorption, declines at the latest Ts. The scatter for the late Ts reflects variation in m/H and g which impacts H2. Mid-IR: A dramatic increase in [3.6]-[4.5] and decrease in [4.5]-[5.8] for the Ts as more flux emerges at [4.5] between strong CH4 and H2O bands. (Leggett et al. in prep, Leggett et al. 2007 ApJ 655, Patten et al. 2006 ApJ 651.) L0 T0 L0 T0 J-H H-K [3.6]-[4.5] [4.5]-[5.8] Brown Dwarfs and Exoplanets Shanghai China July 2009

6. Absolute Magnitudes and Type T0 L0 T0 L0 The near-IR brightening or levelling at late-L to early-T is due to the clearing of the condensate cloud decks from the photosphere. Near-IR absolute fluxes drop more steeply than mid-IR, to late-T types. From T5 to T9 MJ increases by 4 mag cf. 1.5 mag for M[4.5]. One implication is that WISE will find ~6x more 400K dwarfs than VISTA. MH MJ M[3.6] M[4.5] Brown Dwarfs and Exoplanets Shanghai China July 2009

7. Models, Color-Mag Diagrams Models must include mixing of CO/CH4 and N2/NH3 to match observations, especially at 5m and 10m for the strong CO and NH3 bands (e.g. Saumon et al. 2007 ApJ 662). The dotted lines exclude mixing. Solid lines are m/H=0 log g=4, 4.5 and 5.0. Dashed are log g=4.5, m/H=-0.3 and +0.3. The models do well except at [3.6] which is too faint. TeffK: TeffK: MJ MH TeffK: TeffK: M[4.5] M[3.6] Brown Dwarfs and Exoplanets Shanghai China July 2009

8. Colors and Temperature Stephens et al. 2009 astro-ph 0906.2991 The distribution of flux between the near-IR and mid-IR is sensitive to Teff. The near- to mid-IR long wavelength baseline can give a Teff indicator that is insensitive to m/H and g. Warren et al. (2007 MNRAS 381) and Stephens et al. (2009 0906.2991) show that H-[4.5] is a clean indicator, less sensitive to clouds, m/H or g than J or K colors. M8 L2 L6 T6 M8 L2 L6 T6 Brown Dwarfs and Exoplanets Shanghai China July 2009

9. The H-[4.5]>3 Sample We selected all T dwarfs with IRAC photometry with H-[4.5]>3; this includes all T7.5 and later types with IRAC data except: the metal-rich T7.5 2M 1217 (Saumon et al. 2007) is excluded, and the metal-poor T6p 2M 0937 and T6.5e 2M1237 are included using this color selection. Brown Dwarfs and Exoplanets Shanghai China July 2009

10. Metallicity and Gravity Trends • Y-J, J-H, H-K and [4.5]-[5.8] are plotted against H-[4.5]. • H-[4.5]>3 implies the sample has Teff<800K, with some dependency on g and m/H. • Y-J and J-H do not show trends with m/H or g, and the models calculate more dispersion in J-H than is seen. • However H-K and [4.5]-[5.8] do show trends that are consistent with the models, which imply that H-K is more sensitive to metallicity and [4.5]-[5.8] to gravity. • These trends suggest that ULAS 1017 and ULAS 1238 are metal-rich and low-gravity, similar to ULAS 0034 and ULAS 1335. ULAS 1017 has Teff ~750K, ULAS 1238 ~600K. Y-J J-H Metal-rich Solar metals Metal-poor Unknown metals log g = >5 log g = 5 log g = 4-4.5 unknown g [4.5]-[5.8] H-K Brown Dwarfs and Exoplanets Shanghai China July 2009

11. Diversity in Age, Mass, Metallicity 2MASS T7-T8s tend to be high-g low-m/H as they are identified by blue J-H and H-K colors. But why are 4 of 5 ULAS T8-T9s low-g and metal rich (and so young and low-mass)? Brown Dwarfs and Exoplanets Shanghai China July 2009

12. Simulations of the Cold Population Mass Burgasser (2004 ApJS 155) simulates the low-mass population for various mass functions, assuming a constant star formation rate. The median mass decreases to low Teff as low mass dwarfs start and remain cooler than high mass dwarfs. But for a flat mass function, at Teff=600K, the median mass is 30-40 MJup and the median age >6 Gyr, corresponding to log g ~ 5.0. This is significantly more massive than the ULAS T8-9s. Burgasser (2004 ApJS 155) =0.5 40MJ =1.5 20MJ Age =0.5 1000K 500K 6Gyr The colors used for identification of the ULAS T dwarfs, YJH, are apparently not sensitive to metallicity and gravity. There should be no gravity bias due to brightness as brown dwarfs with similar temperatures but different gravities have similar luminosities, as radius is ~constant. This puzzle will be re-examined when the ULAS sample size is larger. 4Gyr 2Gyr =1.5 1000K 500K Brown Dwarfs and Exoplanets Shanghai China July 2009

13. Summary • The wavelength region beyond 3m makes up most of the emitted flux for dwarfs cooler than 700K and is crucial for analysis of their photospheres. This region is extremely challenging from the ground and extensions to the warm Spitzer mission (from 2011 to 2013) and WISE mission (from 2010 to 2011) are desirable. • WISE should be sensitive to 500K dwarfs out to 10, 15 and 10 pc in the 3.3, 4.6 and 12 m bandpasses. For a flat mass function, this all-sky mission should discover around 45 500K brown dwarfs and 30 400K (Y?) dwarfs. These will be bright enough for spectroscopic followup in the near- and mid-IR by JWST. • The long-baseline color HMKO-[4.5]IRAC provides a good indicator of Teff. HMKO-KMKO and [4.5]IRAC-[5.8]IRAC are sensitive to metallicity and gravity. As the models improve it may be possible to separate gravity and metallicity effects. • These colors suggest that 4 of the 5 ULAS T8-T9 dwarfs are young low-gravity and hence low-mass objects: younger than 2 Gyr and less massive than 20 Mjup. This trend is not expected and not currently understood, we will revisit this as the sample size increases and the UKIDSS survey continues. Brown Dwarfs and Exoplanets Shanghai China July 2009

14. Saumon & Marley 2008 Evolutionary Sequences Evolution of brown dwarfs in Teff and gravity for cloudless atmospheres. The evolution proceeds from right to left along the thick solid black lines, which are labeled with the mass in solar masses. Isochrones are shown by the blue dotted lines for 0.01, 0.02, 0.04, 0.1, 0.2, 0.4, 1, 2, 4, and 10 Gyr (right to left), with the thick blue dotted lines for 0.01, 0.1, 1, and 10 Gyr. The nearly vertical red lines are curves of constant luminosity and the green lines show curves of constant radius. Brown Dwarfs and Exoplanets Shanghai China July 2009