We present the discovery of another seven Y dwarfs from the Wide-field Infrared Survey Explorer (WISE). Using these objects, as well as the first six WISE Y dwarf discoveries from Cushing et al., we ...further explore the transition between spectral types T and Y. We present a table that updates the entire stellar and sub-stellar constituency within 8 pc of the Sun, and we show that the current census has hydrogen-burning stars outnumbering brown dwarfs by roughly a factor of six. We use these discoveries and their preliminary distances to place them in the larger context of the solar neighborhood. We present a table that updates the entire stellar and sub-stellar constituency within 8 pc of the Sun, and we show that the current census has hydrogen-burning stars outnumbering brown dwarfs by roughly a factor of six. More detailed monitoring and characterization of these Y dwarfs, along with dedicated searches aimed at identifying more examples, are certainly required.
ABSTRACT
We present Spitzer observations at 3.6 and 4.5 µm and a near-infrared IRTF SpeX spectrum of the irradiated brown dwarf NLTT5306B. We determine that the brown dwarf has a spectral type of L5 ...and is likely inflated, despite the low effective temperature of the white dwarf primary star. We calculate brightness temperatures in the Spitzer wavebands for both the model radius, and Roche Lobe radius of the brown dwarf, and conclude that there is very little day–night side temperature difference. We discuss various mechanisms by which NLTT5306B may be inflated, and determine that while low-mass brown dwarfs (M < 35 MJup) are easily inflated by irradiation from their host star, very few higher mass brown dwarfs are inflated. The higher mass brown dwarfs that are inflated may be inflated by magnetic interactions or may have thicker clouds.
We present an analysis of 23 L dwarfs whose optical spectra display unusual features. Twenty-one were uncovered during our search for nearby, late-type objects using the Two Micron All Sky Survey, ...while two were identified in the literature. The unusual spectral features, notably weak FeH molecular absorption and weak Na I and K I doublets, are attributable to low gravity and indicate that these L dwarfs are young, low-mass brown dwarfs. We use these data to expand the spectral classification scheme for L0- to L5-type dwarfs to include three gravity classes. Most of the low-gravity L dwarfs have southerly declinations and distance estimates within 60 pc. Their implied youth, on-sky distribution, and distances suggest that they are members of nearby, intermediate-age (~10-100 Myr), loose associations such as the b Pictoris moving group, the Tucana/Horologium Association, and the AB Doradus moving group. At an age of 30 Myr and with effective temperatures from 1500 to 2400 K, evolutionary models predict masses of 11-30 M Jupiter for these objects. One object, 2M 0355+11, with J - Ks = 2.52 ± 0.03, is the reddest L dwarf found in the field and its late spectral type and spectral features indicative of a very low gravity suggest it might also be the lowest mass field L dwarf. However, before ages and masses can be confidently adopted for any of these low-gravity L dwarfs, additional kinematic observations are needed to confirm cluster membership.
New Spectral Types L and T Kirkpatrick, J. Davy
Annual review of astronomy and astrophysics,
01/2005, Letnik:
43, Številka:
1
Journal Article
Recenzirano
Odprti dostop
▪ Abstract The establishment of new spectral classes cooler than type M has had a brief, yet already rich, history. Prototypes of the new “L dwarf” and “T dwarf” classes were first found in the late ...1980s to mid-1990s, with a flood of new discoveries occurring in the late 1990s with the advent of deep, large-area, digital sky surveys. Over four hundred and fifty L and T dwarfs are now cataloged. This review concentrates on the spectroscopic properties of these objects, beginning with the establishment of classification schemes rooted in the MK Process. The resulting grid of spectral types is then used as a tool to ferret out the underlying physics. The temperature ranges covered by these spectral types, the complex chemical processes responsible for the shape of their emergent spectra, their nature as either true stars or brown dwarfs, and their number density in the Galaxy are discussed. Two promising avenues for future research are also explored: the extension of the classification system to three dimensions to account for gravity- and metallicity-dependent features, and the capability of newer large-area surveys to uncover brown dwarfs cooler than those now recognized.
Celotno besedilo
Dostopno za:
CMK, DOBA, FMFMET, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
ABSTRACT We present WISEA J114724.10-204021.3, a young, low-mass, high-probability member of the TW Hya association (TWA). WISEA J114724.10-204021.3 was discovered based on its red AllWISE color (W1 ...− W2 = 0.63 mag) and extremely red 2MASS J − KS color (>2.64 mag), the latter of which is confirmed with near-infrared photometry from the Visible and Infrared Survey Telescope for Astronomy Hemisphere Survey (J − KS = 2.57 0.03). Follow-up near-infrared spectroscopy shows a spectral type of L7 1 as well as several spectroscopic indicators of youth. These include a peaked H-band shape and a steeper K-band slope, traits typically attributed to low surface gravity. The sky position, proper motion, and distance estimates of WISEA J114724.10-204021.3 are all consistent with membership in the ∼10 Myr old TWA. Using the age of the TWA and evolutionary models, we estimate the mass of WISEA J114724.10-204021.3 to be 5-13 MJup, making it one of the youngest and lowest-mass free-floating objects yet discovered in the Solar neighborhood.
M subdwarfs are low-metallicity M dwarfs that typically inhabit the halo population of the Galaxy. Metallicity controls the opacity of stellar atmospheres; in metal-poor stars, hydrostatic ...equilibrium is reached at a smaller radius, leading to smaller radii for a given effective temperature. We compile a sample of 88 stars that span spectral classes K7 to M6 and include stars with metallicity classes from solar-metallicity dwarf stars to the lowest metallicity ultra subdwarfs to test how metallicity changes the stellar radius. We fit models to Palomar Double Spectrograph (DBSP) optical spectra to derive effective temperatures (Teff) and we measure bolometric luminosities (Lbol) by combining broad wavelength-coverage photometry with Gaia parallaxes. Radii are then computed by combining the Teff and Lbol using the Stefan-Boltzman law. We find that for a given temperature, ultra subdwarfs can be as much as five times smaller than their solar-metallicity counterparts. We present color-radius and color-surface brightness relations that extend down to Fe/H of −2.0 dex, in order to aid the radius determination of M subdwarfs, which will be especially important for the WFIRST exoplanetary microlensing survey.
Abstract
Using a sample of 361 nearby brown dwarfs, we have searched for 4.6
μ
m variability, indicative of large-scale rotational modulations or large-scale, long-term changes on timescales of over ...10 yr. Our findings show no statistically significant variability in Spitzer’s Infrared Array Camera (IRAC) channel 2 (ch2) or Wide-field Infrared Survey Explorer W2 photometry. For Spitzer the ch2 1
σ
limits are ∼8 mmag for objects at 11.5 mag and ∼22 mmag for objects at 16 mag. This corresponds to no variability above 4.5% at 11.5 mag and 12.5% at 16 mag. We conclude that highly variable brown dwarfs, at least two previously published examples of which have been shown to have 4.6
μ
m variability above 80 mmag, are very rare. While analyzing the data, we also developed a new technique for identifying brown dwarf binary candidates in Spitzer data. We find that known binaries have IRAC ch2 point response function (PRF) flux measurements that are consistently dimmer than aperture flux measurements. We have identified 59 objects that exhibit such PRF versus aperture flux differences and are thus excellent binary brown dwarf candidates.
Abstract
We present the first JWST spectral energy distribution of a Y dwarf. This spectral energy distribution of the Y0 dwarf WISE J035934.06−540154.6 consists of low-resolution (
λ
/Δ
λ
∼100) ...spectroscopy from 1–12
μ
m and three photometric points at 15, 18, and 21
μ
m. The spectrum exhibits numerous fundamental, overtone, and combination rotational–vibrational bands of H
2
O, CH
4
, CO, CO
2
, and NH
3
, including the previously unidentified
ν
3
band of NH
3
at 3
μ
m. Using a Rayleigh–Jeans tail to account for the flux emerging at wavelengths greater than 21
μ
m, we measure a bolometric luminosity of 1.523 ± 0.090 × 10
20
W. We determine a semiempirical effective temperature estimate of
467
−
18
+
16
K using the bolometric luminosity and evolutionary models to estimate a radius. Finally, we compare the spectrum and photometry to a grid of atmospheric models and find reasonably good agreement with a model having
T
eff
= 450 K, log
g
= 3.25 cm s
−2
, and M/H = −0.3. However, the low surface gravity implies an extremely low mass of 1
M
Jup
and a very young age of 20 Myr, the latter of which is inconsistent with simulations of volume-limited samples of cool brown dwarfs.
We present a method for measuring the physical parameters of the coldest T-type brown dwarfs using low-resolution near-infrared spectra. By comparing H sub(2)O and H sub(2)-sensitive spectral ratios ...between empirical data and theoretical atmosphere models, and calibrating these ratios to measurements for the well-characterized 2-5 Gyr companion brown dwarf Gliese 570D, we derive estimates of the effective temperatures and surface gravities for 13 mid- and late-type field T dwarfs. We also deduce the first quantitative estimate of subsolar metallicity for the peculiar T dwarf 2MASS 0937+2931. Derived temperatures are consistent with prior estimates based on parallax and bolometric luminosity measurements, and examination of possible systematic effects indicate that the results are robust. Two recently discovered late-type T dwarfs, 2MASS 0939-2448 and 2MASS 1114-2618, both appear to be 50 K cooler than the latest type T dwarf, 2MASS 0415-0935, and are potentially the coldest and least luminous brown dwarfs currently known. We find that, in general, higher surface gravity T dwarfs have lower effective temperatures and luminosities for a given spectral type, explaining previously observed scatter in the T sub(eff)/spectral type relation for these objects. Masses, radii, and ages are estimated for the T dwarfs in our sample using the evolutionary models of Burrows et al.; we also determine masses and radii independently for eight T dwarfs with measured luminosities. These two determinations are largely consistent, lending support to the validity of evolutionary models at late ages. Our method is well suited to large samples of faint brown dwarfs and can ultimately be used to directly measure the substellar mass function and formation history in the Galaxy.
We present ground-based spectroscopic verification of 6 Y dwarfs (see also Cushing et al.), 89 T dwarfs, 8 L dwarfs, and 1 M dwarf identified by the Wide-field Infrared Survey Explorer (WISE). Eighty ...of these are cold brown dwarfs with spectral types >=T6, six of which have been announced earlier by Mainzer et al. and Burgasser et al. We present color-color and color-type diagrams showing the locus of M, L, T, and Y dwarfs in WISE color space. Near-infrared and, in a few cases, optical spectra are presented for these discoveries. Near-infrared classifications as late as early Y are presented and objects with peculiar spectra are discussed. Using these new discoveries, we are also able to extend the optical T dwarf classification scheme from T8 to T9. After deriving an absolute WISE 4.6 Delta *mm (W2) magnitude versus spectral type relation, we estimate spectrophotometric distances to our discoveries. We also use available astrometric measurements to provide preliminary trigonometric parallaxes to four of our discoveries, which have types of L9 pec (red), T8, T9, and Y0; all of these lie within 10 pc of the Sun. The Y0 dwarf, WISE 1541--2250, is the closest at 2.8+1.3 --0.6 pc; if this 2.8 pc value persists after continued monitoring, WISE 1541--2250 will become the seventh closest stellar system to the Sun. Another 10 objects, with types between T6 and >Y0, have spectrophotometric distance estimates also placing them within 10 pc. The closest of these, the T6 dwarf WISE 1506+7027, is believed to fall at a distance of ~4.9 pc. WISE multi-epoch positions supplemented with positional info primarily from the Spitzer/Infrared Array Camera allow us to calculate proper motions and tangential velocities for roughly one-half of the new discoveries. This work represents the first step by WISE to complete a full-sky, volume-limited census of late-T and Y dwarfs. Using early results from this census, we present preliminary, lower limits to the space density of these objects and discuss constraints on both the functional form of the mass function and the low-mass limit of star formation.
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