Tesfaye Asfaw

11/5/2014

T-Dwarfs

Artist's vision of a T-dwarf

M dwarfL dwarfT dwarfJupiter

~1600 K

~650 K

160 K

~2800 K

INTRODUCTIONT dwarfs are a class of low mass, low temperature(600 –1300 K ), low

luminosity brown dwarfs exhibit spectral signatures of CH4 and H2O 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 Teff ~ 1000 K and luminosity L ~

10-6 Lsun.

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).

M dwarfs are dominated by TiO, VO, H2O, CO absorption plus metal/alkali lines.

L dwarfs replace oxides with hydrides (FeH, CrH, MgH, CaH) and alkalis are prominent.

T dwarfs exhibit strong CH4 and H2O 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 CH4 absorption in their near-infrared spectra.

They are also characterized by Strong H2O and NH3 bands, prominent neutral metal line features, collision-induced H2 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 CH4 absorption bands centered at 1.15, 1.35, 1.65, 2.2, and 3.3μm. There are also strong H2O bands at 1.15, 1.4, and 1.8μm

Red optical

J-band

H-band

K-band

L-band

Mid-Infrared

At mid-infrared wavelengths:Molecular opacity from H2O (5.5–7.0μm), and CH4 (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—H2O, CH4, and CIA H2—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 DataPrimary 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 ComparisonSpectral 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 Teff

Teff 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 featuresdeep and strongly pressure-broadened atomic-line absorptions arisingfrom 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 classificationSDSS J125453.90−012247.4 (To2),2MASS J15031961 + 2525196 (To5), SDSS J162414.37 + 002915.6 (To6), and2MASS J04151954 − 0935066 (To8)

Some Characteristics of the spectra • Steepening of the 8000–10000Å slope.• Strengthening of the 9250Å H2O 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 L7.5 in the optical (Cruz et al. 2003).

It is now known that SDSSp J042348.57 − 041403.5 is a binary likely composed of L6.5 and T2 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/Δλ ≈ 90) spectroscopy over the range 5.3–15.3μm.

These spectra are dominated by three molecular absorbers:

H2O over 6–6.5μm, CH4 at 7.65μm, and NH3 at 10.5μm.

The H2O and CH4 bands increase in strength with later type

NH3 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 H2O, CH4, and NH3 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