TW Hydrae Association Research Articles

Evolution of young brown dwarf disks in the mid-infrared

We have imaged two bona-fide brown dwarfs with TReCS/GEMINI-S and find mid-infrared excess emission that can be explained by optically thick dust disk models. In the case of the young ( ≈ 2 Myr) Cha H α 1 we measure fluxes at 10.4 μ m and 12.3 μ m that are fully consistent with a standard flared disk model and prominent silicate emission. For the ≈ 10 Myr old brown dwarf 2MASS1207-3932 located in the TW Hydrae association we find excess emission at 8.7 μ m and 10.4 μ m with respect to the photosphere, and confirm disk accretion as a likely cause of its strong activity. Disks around brown dwarfs likely last at least as long as their low-mass stellar counterparts in the T-Tauri phase. Grain growth, dust settling, and evolution of the geometry of brown dwarf disks may appear on a timescale of 10 Myr and can be witnessed by observations in the mid-infrared.

Young Stars Near the Sun

▪ Abstract Until the late 1990s the rich Hyades and the sparse UMa clusters were the only coeval, comoving concentrations of stars known within 60 pc of Earth. Both are hundreds of millions of years old. Then beginning in the late 1990s the TW Hydrae Association, the Tucana/Horologium Association, the β Pictoris Moving Group, and the AB Doradus Moving Group were identified within ∼60 pc of Earth, and the η Chamaeleontis cluster was found at 97 pc. These young groups (ages 8–50 Myr), along with other nearby, young stars, will enable imaging and spectroscopic studies of the origin and early evolution of planetary systems.

A giant planet candidate near a young brown dwarf

We present deep VLT/NACO infrared imaging and spectroscopic observations of the brown dwarf 2MASSWJ1207334-393254, obtained during our on-going adaptive optics survey of southern young, nearby associations. This 25 MJup brown dwarf, located 70 pc from Earth, has been recently identified as a member of the TW Hydrae Association (age 8 Myr). Using adaptive optics infrared wavefront sensing to acquire sharp images of its circumstellar environment, we discovered a very faint and very red object at a close separation of 780 mas (55 AU). Photometry in the H, Ks and L' bands and upper limit in J-band are compatible with a spectral type L5-L9.5. Near-infrared spectroscopy is consistent with this spectral type estimate. Different evolutionary models predict an object within the planetary regime with a mass of 5+-2 MJup and an effective temperature of Teff=1250+-200K.

The State of Protoplanetary Material 10 Million years after Stellar Formation: Circumstellar Disks in the TW Hydrae Association

We have used the Spitzer Space Telescope Infrared Spectrograph to observe seven members of the TW Hya association, the nearest stellar association whose age (~10 Myr) is similar to the timescales thought to apply to planet formation and disk dissipation. Only two of the seven targets display infrared excess emission, indicating that substantial amounts of dust still exist closer to the stars than is characteristic of debris disks; however, in both objects we confirm an abrupt short-wavelength edge to the excess, as is seen in disks with cleared-out central regions. The mid-infrared excesses in the spectra of Hen 3-600 and TW Hya include crystalline silicate emission features, indicating that the grains have undergone significant thermal processing. We offer a detailed comparison between the spectra of TW Hya and Hen 3-600, and a model that corroborates the spectral shape and our previous understanding of the radial structure of these protoplanetary disks.

The AB Doradus Moving Group

From radio to X-ray wavelengths, AB Doradus has been an intensively studied star. We have identified ~30 nearby star systems, each with one or more characteristics of youth, that are moving through space together with AB Dor. This diverse set of ~50 million year old star systems is the comoving, youthful group closest to Earth. The group's nucleus is a clustering of a dozen stars ~20 pc from Earth that includes AB Dor itself. The AB Dor moving group joins the previously known and somewhat younger and more distant Tucana/Horologium and TW Hydrae associations and the β Pictoris moving group as excellent laboratories for investigations of forming planetary systems.

Long-lived disc accretion in the η Chamaeleontis pre-main sequence star cluster

High-resolution spectroscopic study of late-type members of the ≈9-L39-old η Chamaeleontis star cluster shows that four stars, RECX 5, 9, 11 and ECHA J0843.3–7905, have broad Hα profiles indicative of ballistic accretion of material from circumstellar discs first identified by virtue of their infrared (IR) excess emission. Quantitative analysis of the profiles finds accretion in η Cha stars at rates comparable to that derived by Muzerolle et al. for members of the similarly aged TW Hydrae Association (TWA); rates 1–3 orders of magnitude lower than in younger classical T Tauri (CTT) stars. Together these studies indicate that the fraction of long-lived inner discs can be significantly higher than that inferred from study of younger pre-main sequence (PMS) populations, which suggest a disc lifetime of <6 Myr. The detection of long-lived discs may have implications for the formation of planetary systems. If slow accretion processes are the dominant formation mechanism for Jovian planets then long-lived discs may be ideal sites to search for evidence for protoplanets.

A Search for Warm Circumstellar Disks in the TW Hydrae Association

A search for previously undetected optically thin disks around stars in the nearby, young, TW Hydrae Association (TWA) was conducted around 16 stars with sensitive 12 and 18 μm photometry. The survey could detect zodiacal-like dust, with temperature 200–300 K, at levels of LIR/L* = 7 × 10-3. Possible mid-infrared excess emission from TWA 17 was detected at the 2 σ level, but none of the other stars showed evidence for circumstellar dust. The rapid disappearance of large amounts of dust around the K- and M-type stars in this sample may mean that any planet formation in the terrestrial planet region was completed very quickly. There appears to be a bimodal distribution of dust disks in TWA, with stars having either copious or negligible warm dust.

Mid-IR Spectroscopy of the Debris Disks in the TW Hydrae Association

Spectroscopic observation between 3-14 μm of 3 stars in the TW Hya Association were obtained using the Aerospace Corporation Broadband Array Spectrograph System (BASS) on the NASA IRTF telescope. The targets observed were TW Hya, HR 4796A, and HD 98800. Both of the late-type stars, TW Hya and HD 98800, exhibit a strong 10 μm silicate emission band. The strong emission indicates the presence of small dust grains close to the star, at angular distances not currently accessible to coronagraphic techniques. The spectral structure due to crystalline material that is present in some young early-type stars (HD 163296, HD 31648, etc.) is not apparent in these two objects. For HR 4796A, the dust emission is weak.

Timescales of Disk Evolution and Planet Formation

It has been suggested that circumstellar disks evolve from dense, actively accreting structures to low-mass, replenished remnants. During this transition, grains may assemble into planetesimals, or the disk may be cleared by newborn planets. Recently identified nearby groups of young stars provide valuable laboratories for probing disk evolution. I discuss the properties of dust disks in the TW Hydrae Association and the MBM 12 cloud, and compare the results to other studies of disk evolution and planet formation timescales.

New Members of the TW Hydrae Association, β Pictoris Moving Group, and Tucana/Horologium Association

We have identified five new members of the TW Hydrae association (TWA), 11 new members of the ? Pic moving group, and 11 new Tucana/Horologium association members. These are the three youngest (30 Myr) known kinematic stellar groups near the Earth. Newly identified ? Pic group members are located mostly in the northern hemisphere, and they have a slightly different U-component of Galactic velocity compared to that of previously known members. Tracing the motion of ? Pic members backward in time for 12 Myr indicates that they might have formed in a small region with an initial velocity dispersion of ~8 km s-1. A couple of mid-M spectral type ? Pic members show emission features [He I ?5876+?6678) and Na D ?5890+?5896)] seen among earlier spectral type stars in the TWA and ? Pic groups. To derive the distances of the non-Hipparcos members of these groups, we have constructed a V-K versus MK color-magnitude diagram that is very useful in separating young K/M stars from older main-sequence counterparts and constraining theoretical pre-main-sequence evolutionary tracks. All newly identified K- and M-type members of the three groups show saturated X-ray activity (LX/Lbol ~ 10-3). One newly identified TWA member, SSS 101727-5354, is estimated to be only 22 pc away from Earth. Its extreme youth, late spectral type (~M5), and proximity to Earth make SSS 101727-5354 perhaps the best target for direct imaging detection of cooling planets.

An Empirical Criterion to Classify T Tauri Stars and Substellar Analogs Using Low-Resolution Optical Spectroscopy

We have compiled and studied photometric and spectroscopic data published in the literature of several star-forming regions and young open clusters (Orion, Taurus, IC 348, Sco-Cen complex, Chamaeleon I, TW Hydrae association, σ Orionis cluster, IC 2391, α Persei cluster, and the Pleiades). Our goal was to seek the definition of a simple empirical criterion to classify stars or brown dwarfs that are accreting matter from a disk on the sole basis of low-resolution optical spectroscopic data. We show that, using Hα equivalent widths and spectral types, we can statistically classify very young stars and brown dwarfs as classical T Tauri stars and substellar analogs. As a boundary between accreting and nonaccreting objects, we use the saturation limit of chromospheric activity at log [L(Hα)/L(bol)] = -3.3 (determined in the open clusters). We discuss the uncertainties in the classification scheme due to the occurrence of flares. We have used this spectroscopic empirical criterion to classify objects found in the literature, and we compute the fraction of accreting objects in several star-forming regions. The fraction of accreting objects appears to decrease from about 50% to about 5% from 1 to 10 Myr for both stars and brown dwarfs.

Flare stars in the TW Hydrae association: the HIP 57269 system

AbstractWe discuss a new member candidate of the TW Hydrae association (TWA) among the stars of the Gershberg et al. (1999) flare star catalog. TWA is one of the closest known associations of young stars at about 60 pc. Three supposedly young flare stars are located in the same region of the sky as TWA. One of them (HIP 57269) shows strong lithium absorption with spectral type K1/K2V and a high level of chromospheric and coronal activity. It is located at a distance of 48.7 ± 6.3 pc in common with the five TWA members observed with Hipparcos (46.7 to 103.9 pc). HIP 57268 A has a wide companion C which also shows lithium absorption at 6707 Å and which has common proper motion with HIP 57269, as well as a close companion resolved visually by Tycho. HIP 57269 A&amp;C lie above the main sequence and are clearly pre‐main‐sequence stars. The UVW‐space velocity is more consistent with the star system being a Pleiades super cluster member. The two other flare stars in the TWA sky region do not show lithium at all and are, hence, unrelated. (© 2003 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Activity and kinematics of members of the TW Hydrae association

We present high-resolution echelle spectroscopy of 20 stars in 16 systems catalogued as members of the TW Hydrae association, and 16 stars identified as possible new members. We have calibrated the range of coronal and chromospheric activity expected for such young stars as a function of spectral type by combining our observations with literature data for field and open cluster stars. We also compute space motions for TWA members and candidate members with proper motion measurements, using two techniques to estimate distances to stars lacking direct trigonometric parallax measurements. The mean space motion of the four TWA members with known parallaxes is (U, V, W; -10.0, -17.8, -4.6) km s - 1 . 14 of the candidates have properties inconsistent with cluster membership; the remaining two are potential new members, although further observations are required to confirm this possibility.

ROSATX‐Ray Spectral Properties of Nearby Young Associations: TW Hydrae, Tucana‐Horologium, and the β Pictoris Moving Group

We present archival ROSAT data for three recently identified, nearby (D < 70 pc), young (~10-40 Myr) stellar associations: the TW Hydrae association, the Tucana-Horologium association, and the β Pictoris moving group. The distributions of ROSAT X-ray hardness ratios (HR1, HR2) for these three groups, whose membership is dominated by low-mass, weak-lined T Tauri stars, are tightly clustered and very similar to one another. The value of HR1 for TW Hya itself—the only bona fide classical T Tauri star in any of the nearby groups—is clearly anomalous among these nearby young stars. We compare the hardness ratio distributions of stars in the three nearby groups with those of T Tauri stars, the Hyades, and main-sequence dwarfs in the field. This comparison demonstrates that the X-ray spectra of F through M stars soften with age and that F and G stars evolve more rapidly in X-ray spectral hardness than do K and M stars. It is as yet unclear whether this trend can be attributed to age-dependent changes in the intrinsic X-ray spectra of stars of type F and later, to a decrease in the column density of circumstellar gas (e.g., in residual protoplanetary disks), or to the diminishing contributions of star-disk interactions to X-ray emission. Regardless, these results demonstrate that analysis of archival ROSAT X-ray spectral data can help both to identify nearby, young associations and to ascertain the X-ray emission properties of members of known associations.

Detection of Crystalline Silicates around the T Tauri Star Hen 3-600A

We have carried out mid-infrared N-band spectroscopic observations of the T Tauri star Hen 3-600A in the TW Hydra association with the COMICS on the 8.2m Subaru Telescope and found structured features in its spectrum. These structured features are well explained by a combination of crystalline forsterite, crystalline enstatite, silica and glassy olivine grains. Among intermediate-mass young stellar objects (YSOs), crystalline silicates have already been detected, but no firm detection has been reported so far for low-mass YSOs such as T Tauri stars. This is the first clear detection of crystalline silicates in low-mass YSOs and shows that the crystallization event occurs even in the protoplanetary disk of low-mass YSOs in the T Tauri phase. The physical processes leading to the inferred dust composition in the Hen3-600A system may be analogous to those occured in the early epoch of the Solar system.

Radial Velocity Survey of Members and Candidate Members of the TW Hydrae Association

We report spectroscopic observations of stars belonging to the young nearby group known as the TW Hydrae association, as well as of a number of potential members of the association identified in kinematic and X-ray surveys. Multiple radial velocity measurements were obtained for each object, several of which turn out to be multiple systems. Orbital solutions are presented for three double-lined binaries, one singlelined binary, and a double-lined triple system, all with short periods. Effective temperatures and projected rotational velocities are presented for each visible object. None of the candidate members of the association in our sample are confirmed as a true member. The large fraction of close binaries among the candidate members has to do with their selection based on X-ray emission from ROSAT, which tends to favor the inclusion of tidally locked systems that are active but not necessarily young.

A Search for Disk Emission in Young Brown Dwarfs: L'-band Observations of σ Orionis and TW Hydrae

Studies of disks around young brown dwarfs are of paramount importance to our understanding of the origin, diversity and early evolution of sub-stellar objects. Here we present first results from a systematic search for disk emission in a spectroscopically confirmed sample of young objects near or below the sub-stellar boundary in a variety of star-forming regions. Our VLT and Keck L'-band observations of the σ Orionis and TW Hydrae associations suggest that if a majority of brown dwarfs are born with disks, at least the inner regions of those disks evolve rapidly, possibly clearing out within a few million years.

Brown Dwarfs and the TW Hydrae Association

I report the results of a survey for low-mass (0.030 <~ M <~ 0.013 M_solar) brown dwarfs in the direction of the TW Hya association using 2MASS. Two late-M dwarfs show signs of low surface gravity and are strong candidates to be young, very-low-mass (M ~ 0.025 M_solar) brown dwarfs related to the TW Hya association. 2MASSW J1207334-393254 is particularly notable for its strong H alpha emission. The number of detected brown dwarfs is consistent with the substellar mass function in richer star formation environments. Newly identified late-M and L dwarfs in the field are also discussed. Unusual objects include an L dwarf with strong H alpha emission, a possible wide M8/M9 triple system, and a possible L dwarf companion to an LHS star.

The TW Hydrae Association: Discovery of T Tauri Star Members Near HR 4796

We have identified six T Tauri stars, at least three of which are double, that are members of the nearest region of recent star formation, the TW Hydrae association. The newly discovered systems are mostly located south and east of and farther from Earth than most previously known members; thus, the full extent of the association is not yet established. Including secondaries in multiple star systems, the association is now known to include more than 30 T Tauri stars and a brown dwarf. We compare the TW Hydrae association to four other young nearby associations.

Silicate Emission in the TW Hydrae Association

The TW Hydrae association is the nearest young stellar association. Among its members are HD 98800, HR 4796A, and TW Hydrae itself, the nearest known classical T Tauri star. We have observed these three stars spectroscopically between 3 and 13 μm. In TW Hya, the spectrum shows a silicate emission feature that is similar to many other young stars' with protostellar disks. The 11.2 μm feature indicative of significant amounts of crystalline olivine is not as strong as in some young stars and solar system comets. In HR 4796A, the thermal emission in the silicate feature is very weak, suggesting little in the way of (small silicate) grains near the star. The silicate band of HD 98800 (observed by us, but also reported by Sylvester & Skinner) is intermediate in strength between TW Hya and HR 4796A.