TW Hydrae Association Members Research Articles

X-shooter spectroscopy of young stars with disks

Context.Measurements of the fraction of disk-bearing stars in clusters as a function of age indicate protoplanetary disk lifetimes ≲10 Myr. However, our knowledge of the time evolution of mass accretion in young stars over the disk lifespans is subject to many uncertainties, especially at the lowest stellar masses (M⋆).Aims.We investigate ongoing accretion activity in young stars in the TW Hydrae association (TWA). The age of the association (∼8–10 Myr) renders it an ideal target for probing the final stages of disk accretion, and its proximity (∼50 pc) enables a detailed assessment of stellar and accretion properties down to brown dwarf masses.Methods.Our sample comprises eleven TWA members with infrared excess, amounting to 85% of the total TWA population with disks. Our targets span spectral types between M0 and M9, and masses between 0.58M⊙and 0.02M⊙. We employed homogeneous spectroscopic data from 300 nm to 2500 nm, obtained synoptically with the X-shooter spectrograph, to derive the individual extinction, stellar parameters, and accretion parameters for each object simultaneously. We then examined the luminosity of Balmer lines and forbidden emission lines to probe the physics of the star–disk interaction environment.Results.Disk-bearing stars represent around 24% of the total TWA population. We detected signatures of ongoing accretion for 70% of our TWA targets for which accurate measurements of the stellar parameters could be derived. This implies a fraction of accretors between 13–17% across the entire TWA (that accounts for the disk-bearing and potentially accreting members not included in our survey). The spectral emission associated with these stars reveals a more evolved stage of these accretors compared to younger PMS populations studied with the same instrument and analysis techniques (e.g., Lupus): first, a large fraction (∼50%) exhibit nearly symmetric, narrow Hαline profiles; second, over 80% of them exhibit Balmer decrements that are consistent with moderate accretion activity and optically thin emission; third, less than a third exhibit forbidden line emission in [O I] 6300 Å, which is indicative of winds and outflows activity; and fourth, only one sixth exhibit signatures of collimated jets. However, the distribution in accretion rates (Ṁacc) derived for the TWA sample closely follows that of younger regions (Lupus, Chamaeleon I,σOrionis) over the mass range of overlap (M⋆ ∼ 0.1–0.3M⊙). An overall correlation betweenṀaccandM⋆is detected and best reproduced by the functionṀacc∝M∝2.1±0.5.Conclusion.At least in the lowestM⋆regimes, stars that still retain a disk at ages ∼8–10 Myr are found to exhibit statistically similar, albeit moderate, accretion levels as those measured around younger objects. This “slow”Ṁaccevolution that is apparent at the lowest masses may be associated with longer evolutionary timescales of disks around low-mass stars, which is suggested by the mass-dependent disk fractions reported in the literature within individual clusters.

THE COOLEST ISOLATED BROWN DWARF CANDIDATE MEMBER OF TWA

We present two new late-type brown dwarf candidate members of the TW Hydrae association (TWA) : 2MASS J12074836-3900043 and 2MASS J12474428-3816464, which were found as part of the BANYAN all-sky survey (BASS) for brown dwarf members to nearby young associations. We obtained near-infrared (NIR) spectroscopy for both objects (NIR spectral types are respectively L1 and M9), as well as optical spectroscopy for J1207-3900 (optical spectral type is L0{\gamma}), and show that both display clear signs of low-gravity, and thus youth. We use the BANYAN II Bayesian inference tool to show that both objects are candidate members to TWA with a very low probability of being field contaminants, although the kinematics of J1247-3816 seem slightly at odds with that of other TWA members. J1207-3900 is currently the latest-type and the only isolated L-type candidate member of TWA. Measuring the distance and radial velocity of both objects is still required to claim them as bona fide members. Such late-type objects are predicted to have masses down to 11-15 MJup at the age of TWA, which makes them compelling targets to study atmospheric properties in a regime similar to that of currently known imaged extrasolar planets.

Kinematic analysis and membership status of TWA22 AB

TWA22 was initially regarded as a member of the TW Hydrae association (TWA). In addition to being one of the youngest (~8Myr) and nearest (~20pc) stars to Earth, TWA22 has proven to be very interesting after being resolved as a tight, very low-mass binary. This binary can serve as a very useful dynamical calibrator for pre-main sequence evolutionary models. However, its membership in the TWA has been recently questioned despite due to the lack of accurate kinematic measurements. Based on proper motion, radial velocity, and trigonometric parallax measurements, we aim here to re-analyze the membership of TWA22 to young, nearby associations. Using the ESO NTT/SUSI2 telescope, we observed TWA22AB during 5 different observing runs over 1.2 years to measure its trigonometric parallax and proper motion. This is a part of a larger project measuring trigonometric parallaxes and proper motions of most known TWA members at a sub-milliarcsec level. HARPS at the ESO 3.6m telescope was also used to measure the system's radial velocity over 2 years. We report an absolute trigonometric parallax of TWA22AB, 57.0mas, corresponding to a distance 17.5pc from Earth. Measured proper motions of TWA22AB are -175.8mas/yr in right ascension and -21.3mas/yr in declination. Finally, from HARPS measurements, we obtain a radial velocity 14.8km/s.

Characterizing the Disk around the TW Hydrae Association Brown Dwarf 2MASSW J1207334−393254

We present detailed modeling of the disk around the TW Hydrae Association (TWA) brown dwarf 2MASSW J1207334-393254 (2M1207), using {\it Spitzer} observations from 3.6 to 24 $\micron$. The spectral energy distribution (SED) does not show a high amount of flaring. We have obtained a good fit using a flat disk of mass between $10^{-4}$ and $10^{-6}$ $M_{\sun}$, $\dot{M}$ $\la10^{-11} M_{\sun}$/yr and a large inclination angle between 60$\degr$ and 70$\degr$. We have used three different grain models to fit the 10 $\micron$ Si emission feature, and have found the results to be consistent with ISM-like dust. In comparison with other TWA members, this suggests lesser dust processing for 2M1207 which could be explained by mechanisms such as aggregate fragmentation and/or turbulent mixing. We have found a good fit using an inner disk radius equal to the dust sublimation radius, which indicates the absence of an inner hole in the disk. This suggests the presence of a small K-$L^{\prime}$ excess, similar to the observed K-[3.6] excess.

HSTNICMOS Imaging of the Planetary‐mass Companion to the Young Brown Dwarf 2MASSW J1207334−393254

Multi-band (0.9 to 1.6 um) images of the TW Hydrae Association (TWA) brown dwarf, 2MASS J1207334-393254 (also known as 2M1207), and its candidate planetary mass companion (2M1207b) were obtained on 2004 Aug 28 and 2005 Apr 26 with HST/NICMOS. The images from these two epochs unequivocally confirm the two objects as a common proper motion pair (16.0 sigma confidence). A new measurement of the proper motion of 2M1207 implies a distance to the system of 59+-7 pc and a projected separation of 46+-5 AU. The NICMOS and previously published VLT photometry of 2M1207b, extending overall from 0.9 to 3.8 um, are fully consistent with an object of a few Jupiter masses at the canonical age of a TWA member (~8 Myr) based on evolutionary models of young giant planets. These observations provide information on the physical nature of 2M1207b and unambiguously establish that the first direct image of a planetary mass companion in orbit around a self-luminous body, other than our Sun, has been secured.

SSSPM J1102-3431: A probable new young brown dwarf member of the TW Hydrae Association

We have used archival survey data, most importantly from the SuperCOSMOS Sky Surveys, to derive accurate proper motions and thus confirm the membership of two previously announced isolated brown dwarfs in the nearby, ∼10 Myr old TW Hydrae Association (TWA), 2MASSW J1207334−393254 and 2MASSW 1139511−315921 (Gizis 2002), and to discover a new substellar candidate. This new object, SSSPM J1102−3431, has extremely red optical, optical-to-infrared, and near- infrared colours (R − I = 3.2, I − J = 3.4, J − Ks = 1.15), while low-resolution not only classifies it as a late-type (M8.5) object, but also shows spectral signatures of low gravity as expected for a young brown dwarf. If it is a true TWA member, brown dwarf cooling models predict its mass to be ∼ 20 MJup. SSSPM J1102−3431 lies only ∼12 arcmin away from the T Tauri star TW Hya and their proper motions agree to within the errors, and thus it is possible that the two objects form an extremely wide binary system, with a separation of ∼40 000 AU at d = 56 pc. We have also obtained improved proper motions for SSS 120727.4−324700 and SSS 101726.7−535428, recent additions to the list of TWA members (Song et al. 2003). The relatively large proper motion of the latter object is consistent with the suggested proximity (d ∼ 22 pc) of this lowest-mass stellar representative of the TWA.

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.

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&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 & Co. KGaA, Weinheim)