The Discovery of a Second Field Methane Brown Dwarf from Sloan Digital Sky Survey Commissioning Data

We report the discovery of a second field methane brown dwarf from the commissioning data of the Sloan Digital Sky Survey (SDSS). The object, SDSS J134646.45-003150.4 (hereafter SDSS 1346-00), was selected because of its very red color and stellar appearance. Its spectrum between 0.8 and 2.5 μm is dominated by strong absorption bands of H2O and CH4 and closely mimics those of Gliese 229B and SDSS 162414.37+002915.6 (hereafter SDSS 1624+00), two other known methane brown dwarfs. SDSS 1346-00 is approximately 1.5 mag fainter than Gliese 229B, suggesting that it lies about 11 pc from the Sun. The ratio of flux at 2.1 μm to that at 1.27 μm is larger for SDSS 1346-00 than for Gliese 229B and SDSS 1624+00, which suggests that SDSS 1346-00 has a slightly higher effective temperature than the others. Based on a search area of 130 deg2 and a detection limit of z*=19.8, we estimate a space density of 0.05 pc-3 for methane brown dwarfs with Teff approximately 1000 K in the 40 pc3 volume of our search. This estimate is based on small-sample statistics and should be treated with appropriate caution.     

Optical spectroscopic classification and membership of young M dwarfs in star-forming regions

The spectral type is a key parameter in calibrating the temperature which is required to estimate the mass of young stars and brown dwarfs. We describe an approach developed to classify low-mass stars and brown dwarfs in the Trapezium Cluster using red optical spectra, which can be applied to other star-forming regions. The classification uses two methods for greater accuracy: the use of narrow-band spectral indices which rely on the variation of the strength of molecular lines with spectral type and a comparison with other previously classified young, low-mass objects in the Chamaeleon I star-forming region. We have investigated and compared many different molecular indices and have identified a small number of indices which work well for classifying M-type objects in nebular regions. The indices are calibrated for young, pre-main-sequence objects whose spectra are affected by their lower surface gravities compared with those on the main sequence. Spectral types obtained are essentially independent of both reddening and nebular emission lines.
Confirmation of candidate young stars and brown dwarfs as bona fide cluster members may be accomplished with moderate resolution spectra in the optical region by an analysis of the strength of the gravity-sensitive Na doublet. It has been established that this feature is much weaker in these very young objects than in field dwarfs. A sodium spectral index is used to estimate the surface gravity and to demonstrate quantitatively the difference between young (1–2 Myr) objects, and dwarf and giant field stars.     

TRACE-derived Temperature and Emission Measure Profiles along Long-lived Coronal Loops: The Role of Filamentation

Near-infrared spectroscopic observations of a sample of very cool, low-mass objects are presented with higher spectral resolution than in any previous studies. Six of the objects are L dwarfs, ranging in spectral class from L2 to L8/9, and the seventh is a methane or T dwarf. These new observations were obtained during commissioning of the near-infrared spectrometer (NIRSPEC), the first high-resolution near-infrared cryogenic spectrograph for the Keck II 10 m telescope on Mauna Kea, Hawaii. Spectra with a resolving power of R approximately 2500 from 1.135 to 1.360 μm (approximately J band) are presented for each source. At this resolution, a rich spectral structure is revealed, much of which is due to blending of unresolved molecular transitions. Strong lines due to neutral potassium (K i) and bands due to iron hydride (FeH) and steam (H2O) change significantly throughout the L sequence. Iron hydride disappears between L5 and L8, the steam bands deepen, and the K i lines gradually become weaker but wider because of pressure broadening. An unidentified feature occurs at 1.22 μm that has a temperature dependence like FeH but has no counterpart in the available FeH opacity data. Because these objects are 3-6 mag brighter in the near-infrared compared with the I band, spectral classification is efficient. One of the objects studied (2MASSW J1523+3014) is the coolest L dwarf discovered so far by the 2 Micron All-Sky Survey (2MASS), but its spectrum is still significantly different from the methane-dominated objects such as Gl 229B or SDSS 1624+0029.     

The DODO survey – II. A Gemini direct imaging search for substellar and planetary mass companions around nearby equatorial and Northern hemisphere white dwarfs

The aim of the Degenerate Objects around Degenerate Objects (DODO) survey is to search for very low-mass brown dwarfs and extrasolar planets in wide orbits around white dwarfs via direct imaging. The direct detection of such companions would allow the spectroscopic investigation of objects with temperatures much lower  (<500 K)  than the coolest brown dwarfs currently observed. These ultra-low-mass substellar objects would have spectral types >T8.5, and so could belong to the proposed Y dwarf spectral sequence. The detection of a planet around a white dwarf would prove that such objects can survive the final stages of stellar evolution and place constraints on the frequency of planetary systems around their progenitors (with masses between 1.5 and 8   M_⊙  , i.e. early B to mid-F). This paper presents the results of a multi epoch J band common proper motion survey of 23 nearby equatorial and Northern hemisphere white dwarfs. We rule out the presence of any common proper motion companions, with limiting masses determined from the completeness limit of each observation, to 18 white dwarfs. For the remaining five targets, the motion of the white dwarf is not sufficiently separated from the non-moving background objects in each field. These targets require additional observations to conclusively rule out the presence of any common proper motion companions. From our completeness limits, we tentatively suggest that  ≲5 per cent  of white dwarfs have substellar companions with   T _eff≳ 500 K  between projected physical separations of 60–200 au.     

Theoretical modelling of optical and IR spectra of brown dwarfs and ultracool dwarfs

The current state of theoretical modelling of the spectra of cool dwarfs and ultracool objects are discussed. Fits of synthetical spectral energy distributions (SEDs) in the optical and IR spectral regions to observed spectra show problems of theoretical modelling. Problems of interpretation of observed spectra are more obvious for the case of L and T dwarfs. Still even for late‐M dwarfs the situation is far from being perfect. Some examples of the .ne analysis of the spectra of ultracool dwarfs are presented. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Atmospheres of brown dwarfs

Brown dwarfs are the coolest class of stellar objects known to date. Our present perception is that brown dwarfs follow the principles of star formation, and that brown dwarfs share many characteristics with planets. Being the darkest and lowest mass stars known makes brown dwarfs also the coolest stars known. This has profound implication for their spectral fingerprints. Brown dwarfs cover a range of effective temperatures which cause brown dwarfs atmospheres to be a sequence that gradually changes from a M-dwarf-like spectrum into a planet-like spectrum. This further implies that below an effective temperature of \(\lesssim \)2,800 K, clouds form already in atmospheres of objects marking the boundary between M-Dwarfs and brown dwarfs. Recent developments have sparked the interest in plasma processes in such very cool atmospheres: sporadic and quiescent radio emission has been observed in combination with decaying X-ray activity indicators across the fully convective boundary.     

The IAC online catalog for M‐, L‐ and T‐type dwarfs

We present an online catalog for dwarfs with spectral types M, L and T. The catalog currently contains 893 objects listed in order of right ascension. For most of them the spectra can be downloaded from the catalog in ASCII or FITS formats. The web address is http://www.iac.es/galeria/ege/catalogo spectral/.     

IZ photometry of L dwarfs and the implications for brown dwarf surveys

The I − Z colour has recently been shown to be a good temperature indicator for M dwarfs. We present the first IZ photometry of a small sample of L dwarfs ranging in spectral type from L0.5V to L6.0V. We find that the I − Z colour is not a good temperature indicator for objects between L1V and L5V, such objects having colours that overlap with mid M dwarfs. We attribute this to the reduction in the strength of the TiO and VO bands in the L dwarfs, which are the dominant opacity source in the I band for late M dwarfs. Beyond L5V, I − Z appears to be a reasonable indicator. This has important implications for the planning of optical surveys for cool objects in clusters and the field. For example, I − Z will cease to be a good method of identifying brown dwarfs in the Pleiades below around 0.04 M_⊙, and at around 0.075 M_⊙ in the Hyades and Praesepe.     

Brown dwarf characterization with EChO

The Exoplanet Characterization Observatory (EChO) is a space mission concept dedicated to the analysis of exoplanet atmospheres using low-resolution spectroscopy in the infrared region between 0.55 and 11 μm. A fraction of its time will be used for ancillary science. We discuss here the prospect of a small survey of L and T-type brown dwarfs. These cold objects show properties comparable to those of giant planets, with the advantage of being brighter and not perturbed by host stars, therefore, they are very useful to understand processes and properties of planet atmospheres. At the same time, brown dwarfs still pose some challenges to stellar models that must include the formation of clouds and sedimentation processes that occur at the low temperatures of these objects. Hence, our aim is to build up an homogeneous catalogue of spectra of brown dwarfs as well as to characterize the spectral variability observed on some of them, which is attributed to the presence of clouds in their atmospheres. We demonstrate that EChO could provide the spectra of brown dwarfs between 1 and 11 μm with enough accuracy to reach these goals. We also present the current number of known brown dwarfs and we suggest a list of possible targets, although future surveys will probably provide better targets at the time of EChO launch if the mission is selected.     

Analysis of the Spectra of Two Pleiades Brown Dwarfs: Teide 1 and Calar 3

Results of the numerical simulation of spectra of the Pleiades brown dwarfs Calar 3 and Teide 1 are discussed. Synthetical spectra were computed for several model atmospheres from a grid of Allard and Hauschildt (1995). From the comparison of computed and observed spectra we have found that: – the spectrum of Teide 1 may be reasonably well fitted by JOLA in the visible region, including the spectral regions around the Li I lines at λλ 670.8 and 812.6 nm. – The structure of absorption bands observed in the spectrum of Teide 1 and Calar 3 depends mainly on T_eff. The dependence on gravity is much weaker. – A comparison of observed and computed spectra shows that effective temperatures of these Pleiades brown dwarfs is T_eff ≈ 2900 K. This revised version was published online in July 2006 with corrections to the Cover Date.     

Grain Formation in Atmospheres of Cool Dwarfs

We have constructed a grid of model atmospheres for cool dwarf stars and brown dwarfs with T_eff ≤ 3000 K that includes (i) an equation of state which accounts for over 600 gas phase species and 1000 liquids and solids, and (ii) the opacities of corundum (Al₂O₃), iron, enstatite (MgSiO₃) and forsterite (Mg₂SiO₄), as well as amorphous carbon and SiC. We confirm earlier findings of Tsuji, Ohnaka & Aoki (1996a) that grains are abundant in the outer photospheric layers of red and brown dwarfs with spectral type later than M8. We identify high temperature condensates including perovskite (CaTiO₃) that depletes the photospheres of important absorbers including TiO, and we confirm the disappearance of TiO bands in the observed spectra of cool dwarfs. This revised version was published online in September 2006 with corrections to the Cover Date.     

Correlated spectral variability in brown dwarfs

Models of brown dwarf atmospheres suggest they exhibit complex physical behaviour. Observations have shown that they are indeed dynamic, displaying small photometric variations over time-scales of hours. Here, I report results of infrared (0.95–1.64 μm) spectrophotometric monitoring of four field L and T dwarfs spanning time-scales of 0.1–5.5 h, the goal being to learn more about the physical nature of this variability. Spectra are analysed differentially with respect to a simultaneously observed reference source in order to remove Earth-atmospheric variations. The variability amplitude detected is typically 2–10 per cent, depending on the source and wavelength. I analyse the data for correlated variations between spectral indices. This approach is more robust than single band or  χ²  analyses, because it does not assume an amplitude for the (often uncertain) noise level (although the significance test still assumes a shape for the noise power spectrum). Three of the four targets show significant evidence for correlated variability. Some of this can be associated with specific features including Fe, FeH, VO and K  i , and there is good evidence for intrinsic variability in H₂O and possibly also CH₄. Yet some of this variability covers a broader spectral range which would be consistent with dust opacity variations. The underlying common cause is plausibly localized temperature or composition fluctuations caused by convection. Looking at the high signal-to-noise ratio stacked spectra, we see many previously identified spectral features of L and T dwarfs, such as K  i , Na  i , FeH, H₂O and CH₄. In particular, we may have detected methane absorption at 1.3–1.4 μm in the L5 dwarf SDSS 0539−0059.     

The UKIDSS–2MASS proper motion survey – I. Ultracool dwarfs from UKIDSS DR4

The UK Infrared Telescope Infrared Deep Sky Survey (UKIDSS) is the first of a new generation of infrared surveys. Here, we combine the data from two UKIDSS components, the Large Area Survey (LAS) and the Galactic Cluster Survey (GCS), with Two-Micron All-Sky Survey (2MASS) data to produce an infrared proper motion survey for low-mass stars and brown dwarfs. In total, we detect 267 low-mass stars and brown dwarfs with significant proper motions. We recover all 10 known single L dwarfs and the one known T dwarf above the 2MASS detection limit in our LAS survey area and identify eight additional new candidate L dwarfs. We also find one new candidate L dwarf in our GCS sample. Our sample also contains objects from 11 potential common proper motion binaries. Finally, we test our proper motions and find that while the LAS objects have proper motions consistent with absolute proper motions, the GCS stars may have proper motions which are significantly underestimated. This is possibly due to the bulk motion of some of the local astrometric reference stars used in the proper motion determination.     

Atmospheric Chemistry in Giant Planets, Brown Dwarfs, and Low-Mass Dwarf Stars: I. Carbon, Nitrogen, and Oxygen

The chemical species containing carbon, nitrogen, and oxygen in atmospheres of giant planets, brown dwarfs (T and L dwarfs), and low-mass stars (M dwarfs) are identified as part of a comprehensive set of thermochemical equilibrium and kinetic calculations for all elements. The calculations cover a wide temperature and pressure range in the upper portions of giant planetary and T-, L-, and M-dwarf atmospheres. Emphasis is placed on the major gases CH₄, CO, NH₃, N₂, and H₂O but other less abundant gases are included. The results presented are independent of particular model atmospheres, and can be used to constrain model atmosphere temperatures and pressures from observations of different gases. The influence of metallicity on the speciation of these key elements under pressure-temperature (P-T) conditions relevant to low-mass object atmospheres is discussed. The results of the thermochemical equilibrium computations indicate that several compounds may be useful to establish temperature or pressure scales for giant planet, brown dwarf, or dwarf star atmospheres. We find that ethane and methanol abundance are useful temperature probes in giant planets and methane dwarfs such as Gl 229B, and that CO₂ can serve as a temperature probe in more massive objects. Imidogen (NH) abundances are a unique pressure-independent temperature probe for all objects. Total pressure probes for warmer brown dwarfs and M dwarfs are HCN, HCNO, and CH₂O. No temperature-independent probes for the total pressure in giant planets or T-dwarf atmospheres are identified among the more abundant C, N, and O bearing gases investigated here.     

An optical spectroscopic HR diagram for low-mass stars and brown dwarfs in Orion

The masses and temperatures of young low-mass stars and brown dwarfs in star-forming regions are not yet well established because of uncertainties in the age of individual objects and the spectral type–temperature scale appropriate for objects with ages of only a few Myr. Using multi-object optical spectroscopy, 45 low-mass stars and brown dwarfs in the Trapezium Cluster in Orion have been classified and 44 of these confirmed as bona fide cluster members. The spectral types obtained have been converted to effective temperatures using a temperature scale intermediate between those of dwarfs and giants, which is suitable for young pre-main-sequence objects. The objects have been placed on a Hertzsprung–Russell (HR) diagram overlaid with theoretical isochrones. The low-mass stars and the higher mass substellar objects are found to be clustered around the 1 Myr isochrone, while many of the lower mass substellar objects are located well above this isochrone. An average age of 1 Myr is found for the majority of the objects. Assuming coevality of the sources and an average age of 1 Myr, the masses of the objects have been estimated and range from  0.018 to 0.44 M_⊙  . The spectra also allow an investigation of the surface gravity of the objects by measurement of the sodium doublet equivalent width. With one possible exception, all objects have low gravities, in line with young ages, and the Na indices for the Trapezium objects lie systematically below those of young stars and brown dwarfs in Chamaeleon, suggesting that the 820 nm Na index may provide a sensitive means of estimating ages in young clusters.     

Spectral classification of Pleiades brown dwarf candidates

We report on the results of the spectroscopy of 10 objects previously classified as brown dwarf candidates via RIJHK colors by Eisenbeiss et al. (2009), who performed deep imaging observations on a ∼0.4 sq.deg. field at the edge of the Pleiades. We describe and judge on classification techniques in the region of M‐type stars. To classify and characterise the objects, visual and near infrared spectra have been obtained with VLT FORS and ISAAC. The spectral classification was performed using the shape of the spectra as well as spectral indices that are sensitive to the spectral type and luminosity class of M‐type stars and late M‐type brown dwarfs. Furthermore a spectrophotometric distance was calculated and compared the distance of the Pleiades to investigate the membership probability. As a second argument we analyzed the proper motion. The brown dwarf candidates were found not to be brown dwarfs, but late‐K to mid‐M‐type dwarf stars. Based on the obtained distance and tabulated proper motions we conclude that all objects are background dwarf stars (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The metallicity of CM Draconis

We compare observations of the eclipsing binary system CM Draconis (hereafter CM Dra) with synthetic spectra computed using the stellar atmosphere code phoenix . High-resolution infrared spectroscopic observations of six 0.05-μm-wide regions between 1.51 and 2.45 μm, combined with previous work, particularly CM Dra's accurately known surface gravity, enable us to estimate its metallicity using detailed spectral synthesis. We find significant discrepancies between the observed and synthetic spectra throughout most of the region emphasizing the need for higher quality atomic data in the infrared. Nevertheless, the     CO bands beyond 2.3 μm seem to be well modelled and metal-sensitive, and thus high-resolution spectra should be a most powerful diagnostic tool for spectroscopic analyses for M dwarfs and brown dwarfs. The CO bands indicate a metallicity of around −1 dex for CM Dra. This result is supported by observations of two M dwarfs of similar spectral type, GJ 699 (Barnard's star) and GJ 725B. This result supports inferences from previous infrared work, although it does not agree with standard evolutionary models or optical analyses, which both suggest an abundance for CM Dra close to that of the Sun.     

A new population of brown dwarfs

In this paper we report the first results from a survey for low-mass stars and brown dwarfs, based on a photographic stack of around 100 Schmidt plates. This survey extends photographic searches by about 2 mag, and covers an area of 25 deg². Some 30 faint objects with large R − I colours were selected for further study, and were found to have very strong molecular absorption in their spectra, but only moderately red infrared colours. Five of these stars were selected for a parallax programme; three of these were found to be at a distance of around 45 pc, implying a very low luminosity. On the basis of their luminosity alone it is clear that these stars are field brown dwarfs, and we discuss their likely evolutionary status in the context of current models of low-mass stellar evolution.     

红亚矮星研究进展

红亚矮星是甚小质量恒星中的贫金属成员,质量从约0.5M⊙(M⊙为太阳质量)到H燃烧的最小质量(0:075M⊙0:085M⊙,取决于金属丰度),其寿命长于哈勃年龄,是银河系结构和化学增丰史的重要示踪体。与银盘上数量最多的恒星成员红矮星不同,红亚矮星在太阳附近非常稀少,并且其运动学特征与盘矮星有较大差异,属于年老银河系星族,即为年老盘星族、厚盘星族或晕星族。观测上,红亚矮星可以根据其不同于红矮星的自行、测光和光谱特征被识别和证认。由于其恒星表面大气温度很低,并且颜色比同质量的矮星更蓝,因此红亚矮星在赫罗图上位于主序带末端的下方,介于矮星与白矮星之间。红亚矮星的光学波段光谱由金属氧化物(如TiO和VO)和氢化物(如CaH和H2O)的分子吸收带占主导。红亚矮星可按其光谱形态和分子带特征分成不同的光谱型和金属丰度等级,其中晚M型到早L型的亚矮星既可能是小质量的恒星,也可能是较大质量的年轻褐矮星。介绍了对红亚矮星研究的历史背景和前沿动态,详细阐述了光谱分析方法在研究亚矮星中的重要性,以及根据光谱特征对亚矮星进行分类的方法。最后,总结了甚小质量恒星大气模型的发展过程,并探讨了如何利用模型对亚矮星的大气参数进行估算等热点问题。     

The Temperature-Dependent Spectrum of Methane Ice I between 0.7 and 5 μm and Opportunities for Near-Infrared Remote Thermometry

New infrared absorption coefficient spectra of pure methane ice I were measured at temperatures between 30 and 90 K, over wavelengths from 0.7 to 5 μm, along with spectra of methane ice II at 20 K and liquid methane at 93 K. The spectra were derived from transmission measurements through monocrystalline samples grown in a series of closed cells having interior dimensions ranging from 100 μm to 1 cm. The thicker samples permitted measurement of extremely weak absorption bands, with absorption coefficients as small as 0.003 cm⁻¹. We report 14 new absorption bands, which we tentatively assign to specific vibrational transitions. Two of the new bands are attributed to CH₃D. Measurements of the weaker CH₄ bands are particularly needed for interpreting spectral observations of Pluto and Triton, where a number of weak CH₄-ice absorption bands have been observed. The data presented in this paper complement studies of spectral transmission by thin films of methane ice, which are most suitable for measuring the stronger absorption bands. Temperature-dependent spectral features revealed by the new data offer the opportunity to determine CH₄-ice temperatures remotely, via near-infrared reflectance spectroscopy. This approach could prove particularly valuable for future spacecraft exploration of Pluto.