M dwarf L dwarf T dwarf Jupiter ~1600 K ~650 K 160 K ~2800 K
INTRODUCTION T dwarfs are a class of low mass, low temperature(600 –1300 K ), low luminosity brown dwarfs exhibit spectral signatures of CH 4 and H 2 O in the near-infrared. would appear reddish, or magenta, to the eye. Have long lifetime It is thought that there may be a huge number of these. The first identified was Gliese 229B (in 1995), which orbits the red dwarf Gliese 229A. about 2% to 5% the mass of the Sun, or about 20 to 50 times the mass of Jupiter. has an effective temperature T eff ~ 1000 K and luminosity L ~ 10 -6 L sun.
T dwarfs, sometimes called methane dwarfs are characterized by the methane absorption seen in the near-IR that gets progressively stronger as one progresses through the subclasses, making the J-H and H-K colors bluer. In the optical, the spectrum is affected by collisionally induced molecular hydrogen absorption and FeH. There are about 355 T dwarfs known. An up-to-date list is maintained by Chris Gelino and collaborators on the DwarfArchives website (http://dwarfarchives.org).http://dwarfarchives.org
M dwarfs are dominated by TiO, VO, H 2 O, CO absorption plus metal/alkali lines. L dwarfs replace oxides with hydrides (FeH, CrH, MgH, CaH) and alkalis are prominent. T dwarfs exhibit strong CH 4 and H 2 O and extremely broadened Na I and K I. M, L, and T Dwarfs in the IR
This presentation tries to cover spectral characteristics of the T-dwarf class, and current methods for classification in the near-infrared (1–2.5μm), red-optical (0.6–1.0μm), and mid-infrared (5–15μm) wavebands, based largely on low resolution data.
Spectral Classification Subclasses, based on the complete near infrared flux T0-to-T8 spectral standards of Burgasser et al. (2006) and T9 and Y0 standards defined by Cushing et al. (2011). T-Dwarfs are distinguished from L dwarfs (and indeed all other stellar classes) by the presence of CH 4 absorption in their near- infrared spectra. They are also characterized by Strong H 2 O and NH 3 bands, prominent neutral metal line features, collision-induced H 2 absorption, and spectral energy distributions that are increasingly peaked at near-infrared and mid-infrared wavelengths
T DWARF SPECTRAL CHARACTERISTICS The spectrum of a typical mid-type T dwarf The primary distinguishing spectral characters of T dwarfs are the near- infrared CH 4 absorption bands centered at 1.15, 1.35, 1.65, 2.2, and 3.3μm. There are also strong H 2 O bands at 1.15, 1.4, and 1.8μm
The earliest-type T dwarfs exhibit CO absorption at 2.3μm, although this feature weakens and is absent in the mid- and late-type T dwarfs, as predicted by chemical equilibrium models Collision-Induced Absorption by H 2 An additional molecular opacity source present in the near-infrared spectra of T dwarfs is collision-induced H 2 absorption CIA H 2 produces a broad but featureless “band” centered near 2.3μm, along with weaker absorptions at 0.8 and 1.3μm. The 2.3μm band is largely responsible for the blue near-infrared colors of mid- and late- type T dwarfs, in addition to the strong H 2 O and CH 4 features. CIA H 2 absorption is also pressure-sensitive, and as such serves as a useful surface gravity diagnostic.
Atomic line absorption at near-infrared wavelengths is largely limited to K I lines present in the 1.1–1.25μm region, disappear in the spectra of the latest-type T dwarfs. are considerably pressure-broadened
Molecular H 2 O absorption over 0.925–0.98μm present in all of the T dwarf spectra FeH, weakly present in the spectra of mid-type T dwarfs but not in in early- and late- type T dwarf spectra. some evidence for weak CH 4 absorption at 0.89μm common feature in planetary spectra Coincidence with the 0.8943Å Cs I line and possible H 2 O absorption in this region makes this identification uncertain.
At mid-infrared wavelengths: Molecular opacity from H 2 O (5.5–7.0μm), and CH 4 (7.0–9.5μm) dominate
NEAR-INFRARED CLASSIFICATION optical spectra of T dwarfs are exceedingly faint there are far fewer features available for classification considerably brighter in the near-infrared classification is based on near-infrared data. the primary molecular absorbers—H 2 O, CH 4, and CIA H 2 —would be essential in defining a sequence based primarily on low resolution data (λ/Δλ≈100–400) spanning the 1–2.5μm near- infrared
Characteristics of Near-Infrared Classification Data Primary Standards Nine primary spectral standards spanning subtypes T0–T8. Criteria: reasonably bright, to facilitate observation at a variety of telescopes; not known to be spectroscopically peculiar; not known to be significantly variable from photometric or spectroscopic observations; not known to be a resolved multiple system, to the limit of high-angular resolution imaging within ∼ 25◦ of the ecliptic, to facilitate observation from both Northern and Southern hemispheres.
have nearly identical near-infrared spectral energy distributions as the primary standards but are well-separated on the sky to facilitate the observation of a spectral comparison at any time of the year. Do not strictly adhere to the constraints listed above. Alternate Standards
Methods of Classification Direct Spectral Comparison Spectral Indices
Direct Spectral Comparison Compare the near infrared spectral data to equivalent data for the spectral standards after normalizing to the flux peak at 1.27μm. Data obtained over wavelength range (nominally 1–2.5μm) and resolution (λ/Δλ ≈ 150–400) Enable immediate classification of “normal” T dwarfs (e.g., Figure 10.11) that fit within the standard sequence.
Spectral Indices Use ratios of the average or integrated flux density measured at different wavelengths on a spectrum. These indices are defined as the ratio of the integrated flux over a waveband located within an absorption feature to the integrated flux over an identically sized waveband (in wavelength units) in the neighboring pseudocontinuum. Preferred in some situations as an estimator for spectral type. improve signal-to-noise ratio of noisy spectral data. automated classification of very large datasets. smaller index values correspond to stronger absorption.
The five spectral indices recommended by Burgasser et al. (2006a) to estimate T-dwarf subtypes.
Alternate Near-Infrared Classification Schemes Alternate schemes o may be preferable when data have higher resolution or wavelength coverage is limited. o Examined for a sample of 16 T dwarfs with λ/Δλ ≈ 2000 near infrared spectral data in the 1.15– 1.35μm range. o resolve several of the atomic lines present in T-dwarf near-infrared spectra.
Near-Infrared Spectral Types and Physical Parameters In general, later spectral types correspond to fainter absolute magnitudes (particularly for the mid- and late-type T dwarfs in the K-band) At the J-band, early-type T dwarfs appear to be as bright as late-type L dwarfs. This effect known as the “J band bump” The J-band bump appears to be related to the depletion of photospheric condensate dust clouds across the L-dwarf/T-dwarf transition
The spectral types of T dwarfs typed ∼ T4 and later appear to correlate well with T eff T eff as a function of spectral type, based on luminosity determinations from parallax and broadband photometric measurements Consistent with the expectation that stronger molecular bands should be found in cooler photospheres. However, the early-type T dwarfs appear to have roughly the same temperatures as the late-type L dwarfs. has important implications for the evolution of condensate clouds across the L/T transition.
OPTICAL CLASSIFICATION Classification of T dwarfs at red-optical wavelengths provide a continuum of classifications from the M dwarfs through the T dwarfs, and map the properties of dwarfs spanning the L-dwarf/T-dwarf transition, The red-optical spectra of T dwarfs are of interest in their own right. Important features deep and strongly pressure-broadened atomic-line absorptions arising from the 5890/5896Å Na I and 7665/7699Å K I doublets
Difficulty : extreme faintness of T dwarfs at these wavelengths. limited to the brightest examples. required significant investments in 8–10m telescope time to acquire sufficient data.
Spectral Standards Four standards define the optical classification SDSS J125453.90−012247.4 (T o 2), 2MASS J15031961 + 2525196 (T o 5), SDSS J162414.37 + 002915.6 (T o 6), and 2MASS J04151954 − 0935066 (T o 8) Some Characteristics of the spectra Steepening of the 8000–10000Å slope. Strengthening of the 9250Å H 2 O band Weakening of the 8521 and 8943Å Cs I lines. FeH and CrH bands at 8611 and 8692Å decline from L8 to TO2, and are absent from TO5 the 9896Å Wing–Ford band strengthens somewhat from L8 to To5, only to fade again in the later T dwarfs
Classification Methods Direct Spectral Comparison T-dwarf red-optical spectra can again be accomplished via direct comparison to the spectra of the standards listed above.
logarithmic scale permits simultaneous examination of steep spectral slopes with weaker molecular and atomic features. The coarse sampling of the standard grid implies that intermediate types are generally limited to integer subclasses, as opposed to the half- integer subclass resolution of the near-infrared scheme.
Spectral Indices Burgasser et al. (2003b) identified five spectral indices useful for this purpose
Comparison of the optical to Near-Infrared Classification Optical and near-infrared spectral morphologies for T dwarfs are well correlated(within the 1 subtype) For L-dwarf optical and near-infrared types differ by several subclasses. However, agreement is not universal. T dwarf SDSSp J042348.57 − 041403.5, classified T0 in the near-infrared (Geballe et al. 2002) but L 7.5 in the optical (Cruz et al. 2003). It is now known that SDSSp J042348.57 − 041403.5 is a binary likely composed of L 6.5 and T 2 components. Other systems with different optical and near-infrared types have similarly been identified as binaries or binary candidates.
MID-INFRARED CLASSIFICATION T-dwarfs are: extremely faint at shorter wavelengths they are well-detected at longer wavelengths(particularly in the 4–10μm mid-infrared region) Very difficult to observe from the ground due to strong telluric absorption and thermal backgrounds. Observations in this region from the ground is limited primarily to 3–5μm. Spitzer Space Telescope in 2003, with instrumentation sensitive over 3.5 to 160μm, has provided an opportunity to explore the mid-infrared properties of cool dwarf stars and brown dwarfs.
Characteristics of Mid-Infrared Classification Data Spitzer’s IRS instrument provides low resolution (λ/ Δ λ ≈ 90) spectroscopy over the range 5.3–15.3μm. These spectra are dominated by three molecular absorbers: H 2 O over 6–6.5μm, CH 4 at 7.65μm, and NH 3 at 10.5μm. The H 2 O and CH 4 bands increase in strength with later type NH 3 is faintly present at T0 and strengthens considerably through the sequence shown. These three bands therefore appear to map out a sequence that is qualitatively consistent with the near-infrared classifications.
Spectral Indices Three indices are defined to measure the relative strengths of the primary H 2 O, CH 4, and NH 3 molecular bands. Note that the indices are defined in an inverse manner to the one listed for near- infrared and optical data; In this case stronger absorption leads to larger index values.
ADDITIONAL CONSIDERATIONS FOR T-DWARF CLASSIFICATION T dwarfs, whose spectra do not appear to conform to the smooth sequence defined by the standards, have been identified. This could likely arise from three primary sources: unresolved multiplicity, surface gravity/metallicity effects, and condensate cloud effects
References Stellar Spectral Classification, Richard O. Gray & Christopher J. Corbally The spectra of t dwarfs. I. Near-infrared data and spectral classification, Adam J. Burgasser et al. 2002 ApJ 564 421 Toward Spectral Classification of L and T Dwarfs: Infrared and Optical Spectroscopy and Analysis,T. R. Geballe et al. 2002 ApJ 564 466