No abstract

A young, nearby compact aggregate of X-ray emitting pre-main sequence stars was recently discovered in the vicinity of η Chamaeleontis (Mamajek, Lawson & Feigelson 1999, ApJ, 516, L77). In this paper, we further investigate this cluster: its membership, its environs and origins. ROSAT High-Resolution Imager X-ray data for the cluster's T Tauri stars show high levels of magnetic activity and variability. The cluster has an anomalous X-ray luminosity function compared to other young clusters, deficient in stars with low, but detectable X-ray luminosities. This suggests that many low-mass members have escaped the surveyed core region. Photographic photometry from the USNO-A2.0 catalog indicates that additional, X-ray-quiet members exist in the cluster core region. The components of the eclipsing binary RS Cha, previously modeled in the literature as post-main sequence with discordant ages, are shown to be consistent with being coeval pre-MS stars.We compute the Galactic motion of the cluster from Hipparcos data, and compare it to other young stars and associations in the fourth Galactic quadrant. The kinematic study shows that the η Cha cluster, as well as members of the TW Hya association and a new group near ǫ Cha, probably originated near the giant molecular cloud complex that formed the two oldest subgroups of the Sco-Cen OB association roughly 10-15 Myr ago. Their dispersal is consistent with the velocity dispersions seen in giant molecular clouds. A large H I filament and dust lane located near η Cha has been identified as part of a superbubble formed by Sco-Cen OB winds and supernova remnants. The passage of the superbubble may have terminated star-formation in the η Cha cluster and dispersed its natal molecular gas.

We present the outcomes of a Chandra X-Ray Observatory snapshot study of five nearby Herbig Ae/Be (HAeBe) stars that are kinematically linked with the Oph-Sco-Cen association (OSCA). Optical photometric and spectroscopic follow-up was conducted for the HD 104237 field. The principal result is the discovery of a compact group of pre-main-sequence (PMS) stars associated with HD 104237 and its codistant, comoving B9 neighbor Chamaeleontis AB. We name the group after the most massive member. The group has five confirmed stellar systems ranging from spectral type B9 to M5, including a remarkably high degree of multiplicity for HD 104237 itself. The HD 104237 system is at least a quintet, with four low-mass PMS companions in nonhierarchical orbits within a projected separation of 1500 AU of the HAeBe primary. Two of the low-mass members of the group are actively accreting classical T Tauri stars. The Chandra observations also increase the census of companions for two of the other four HAeBe stars, HD 141569 and HD 150193, and identify several additional new members of the OSCA. We discuss this work in light of several theoretical issues: the origin of X-rays from HAeBe stars; the uneventful dynamical history of the high-multiplicity HD 104237 system; and the origin of the Cha group and other OSCA outlying groups in the context of turbulent giant molecular clouds. Together with the similar Cha cluster, we paint a portrait of sparse stellar clusters dominated by intermediate-mass stars 5-10 Myr after their formation.

We present a comprehensive investigation of the ǫ Chamaeleontis association (ǫ Cha), one of several young moving groups spread across the southern sky. We re-assess the putative membership of ǫ Cha using the best-available proper motion and spectroscopic measurements, including new ANU 2.3-m/WiFeS observations. After applying a kinematic analysis our final membership comprises 35-41 stars from B9 to mid-M spectral types, with a mean distance of 110 ± 7 pc and a mean space motion of (U, V, W ) = (−10.9 ± 0.8, −20.4 ± 1.3, −9.9 ± 1.4) km s −1 . Theoretical evolutionary models suggest ǫ Cha is 3-5 Myr old, distinguishing it as the youngest moving group in the solar neighbourhood. Fifteen members show 3-22 µm spectral energy distributions attributable to circumstellar discs, including 11 stars which appear to be actively accreting. ǫ Cha's disc and accretion fractions (29 +8 −6 and 32 +9 −7 per cent, respectively) are both consistent with a typical 3-5 Myr-old population. Multi-epoch spectroscopy reveals three M-type members with broad and highly-variable Hα emission as well as several new spectroscopic binaries. We reject 11 stars proposed as members in the literature and suggest they may belong to the background Cha I and II clouds or other nearby young groups. Our analysis underscores the importance of a holistic and conservative approach to assigning young stars to kinematic groups, many of which have only subtly different properties and ill-defined memberships. We conclude with a brief discussion of ǫ Cha's connection to the young open cluster η Cha and the Scorpius-Centaurus OB association (Sco-Cen). Contrary to earlier studies which assumed η and ǫ Cha are coeval and were born in the same location, we find the groups were separated by ∼30 pc when η Cha formed 4-8 Myr ago in the outskirts of Sco-Cen, 1-3 Myr before the majority of ǫ Cha members.

We have produced brightness and magnetic field maps of the surfaces of CV Cha and CR Cha: two actively accreting G‐ and K‐type T Tauri stars in the Chamaeleon I star‐forming cloud with ages of 3–5 Myr. Our magnetic field maps show evidence for strong, complex multipolar fields similar to those obtained for young rapidly rotating main‐sequence stars. Brightness maps indicate the presence of dark polar caps and low‐latitude spots – these brightness maps are very similar to those obtained for other pre‐main‐sequence and rapidly rotating main‐sequence stars. Only two other classical T Tauri stars have been studied using similar techniques so far: V2129 Oph and BP Tau. CV Cha and CR Cha show magnetic field patterns that are significantly more complex than those recovered for BP Tau, a fully convective T Tauri star. We discuss possible reasons for this difference and suggest that the complexity of the stellar magnetic field is related to the convection zone; with more complex fields being found in T Tauri stars with radiative cores (V2129 Oph, CV Cha and CR Cha). However, it is clearly necessary to conduct magnetic field studies of T Tauri star systems, exploring a wide range of stellar parameters in order to establish how they affect magnetic field generation, and thus how these magnetic fields are likely to affect the evolution of T Tauri star systems as they approach the main sequence.

The formation of planets is directly linked to the evolution of the circumstellar (CS) disk from which they are born. The dissipation timescales of CS disks are, therefore, of direct astrophysical importance in evaluating the time available for planet formation. We employ Spitzer Space Telescope spectra to complete the CS disk census for the late-type members of the ≃ 8 Myr-old η Chamaeleontis star cluster. Of the 15 K-and M-type members, eight show excess emission. We find that the presence of a CS disk is anti-correlated with binarity, with all but one disk associated with single stars. With nine single stars in total, about 80% retain a CS disk. Of the six known or suspected close binaries the only CS disk is associated with the primary of RECX 9. No circumbinary disks have been detected. We also find that stars with disks are slow rotators with surface values of specific angular momentum j = 2 − 15j ⊙ . All high specific angular momentum systems with j = 20 − 30j ⊙ are confined to the primary stars of binaries. This provides novel empirical evidence for rotational disk locking and again demonstrates the much shorter disk lifetimes in close binary systems compared to single star systems. We estimate the characteristic mean disk dissipation timescale to be ∼ 5 Myr and ≈ 9 Myr for the binary and single star systems, respectively.

We present IRS spectra and revised MIPS photometry for the 18 members of the η Chamaeleontis cluster. Aged 8 Myr, the η Cha cluster is one of the few nearby regions within the 5-10 Myr age range, during which the disk fraction decreases dramatically and giant planet formation must come to an end. For the 15 low-mass members, we measure a disk fraction ∼50%, high for their 8 Myr age, and 4 of the 8 disks lack near-IR excesses, consistent with the empirical definition of "transition" disks. Most of the disks are comparable to geometrically flat disks. The comparison with regions of different ages suggests that at least some of the "transition" disks may represent the normal type of disk around low-mass stars. Therefore, their flattened structure and inner holes may be related to other factors (initial masses of the disk and the star, environment, binarity), rather than to pure time evolution. We analyze the silicate dust in the disk atmosphere, finding moderate crystalline fractions (∼10-30%) and typical grain sizes ∼1-3 µm, without any characteristic trend in the composition. These results are common to other regions of different ages, suggesting that the initial grain processing occurs very early in the disk lifetime (<1 Myr). Large grain sizes in the disk atmosphere cannot be used as a proxy for age, but are likely related to higher disk turbulence. The dust mineralogy varies between the 8-12 µm and the 20-30 µm features, suggesting high temperature dust processing and little radial mixing. Finally, the analysis of IR and optical data on the B9 star η Cha reveals that it is probably surrounded by a young debris disk with a large inner hole, instead of being a classical Be star.

We present a study of calibrated low-resolution spectra of the 18 known primaries of the ≈9-Myr-old η Chamaeleontis (η Cha) pre-main-sequence (PMS) star cluster. Using synthetic broad-band colours and narrow-band continuum-sensitive, temperature-sensitive and gravitysensitive indices derived from the spectra, we compare the η Cha stars to standard dwarfs. We find that the VRI colours of the PMS stars are indistinguishable from those of mainsequence stars, but that a B-band excess attaining ≈0.2 mag for late-M cluster stars is present, which might be an indicator of gravity, metallicity and/or activity differences between the two samples of stars. The narrow-band spectral indices for the η Cha stars possibly indicate higher metallicity and strongly indicate lower surface gravity than the dwarf indices, consistent with the elevated location of the cluster in the Hertzsprung-Russell evolutionary diagram. Using the derived synthetic colours and indices, we adopt spectral types for the late-type η Cha stars. We then produce a table of absolute optical magnitudes and colours representing the cluster isochrone for comparison with PMS evolutionary models. From our results we also conclude that the η Cha stars are unreddened, consistent with the group being a sample of older PMS stars distant from obscuring molecular clouds, except for the A1 member HD 75505 for which we confirm A V = 0.4 mag likely due to the presence of circumstellar material. the gravity-sensitive K I and Na I lines are weaker in young Pleiades brown dwarfs than in field objects of similar spectral type, while Mohanty et al. (2004) have demonstrated the gravity sensitivity of the TiO molecular bands, which increase in strength with decreasing effective gravity, in samples of PMS stars in Upper Scorpius and Taurus star-forming regions. The differences in these features are relevant to all young PMS populations, with ages several Gyr less than old disc dwarfs and elevated in the Hertzsprung-Russell (HR) diagram by several magnitudes above the main sequence.Nearby young clusters have distinct advantages for the study of fundamental properties of PMS stars. They provide a sample of stars across a wide range of spectral types at essentially uniform age, distance and metallicity, and are sufficiently bright for precise spectroscopic characterization. Our laboratory is the ≈9-Myr-old η Chamaeleontis (η Cha) cluster (Mamajek, Lawson & Feigelson 1999). The cluster is an ideal PMS group for the study of 'intermediate-aged' PMS stars owing to its well-defined distance from Hipparcos measurements (d = 97 ± 3 pc), compactness (extent ∼1 pc), the apparent high degree of coevality of the stellar C 2004 RAS