* Astronomy

Blobrana · L

RE: T Chamaeleontis

Blobrana · L

Title: Sculpting the disk around T Cha: an interferometric view
Authors: Johan Olofsson, Myriam Benisty, Jean-Baptiste Le Bouquin, Jean-Philippe Berger, Sylvestre Lacour, François Ménard, Thomas Henning, Aurélien Crida, Leonard Burtscher, Gwendolyn Meeus, Thorsten Ratzka, Christophe Pinte, Jean-Charles Augereau, Fabien Malbet, Bernard Lazareff, Wesley A. Traub

Circumstellar disks are believed to be the birthplace of planets and are expected to dissipate on a timescale of a few Myr. The processes responsible for the removal of the dust and gas will strongly modify the radial distribution of the dust and consequently the SED. In particular, a young planet will open a gap, resulting in an inner disk dominating the near-IR emission and an outer disk emitting mostly in the far-IR. We analyse a full set of data (including VLTI/Pionier, VLTI/Midi, and VLT/NaCo/Sam) to constrain the structure of the transition disk around TCha. We used the Mcfost radiative transfer code to simultaneously model the SED and the interferometric observations. We find that the dust responsible for the emission in excess in the near-IR must have a narrow temperature distribution with a maximum close to the silicate sublimation temperature. This translates into a narrow inner dusty disk (0.07-0.11 AU). We find that the outer disk starts at about 12 AU and is partially resolved by the Pionier, Sam, and Midi instruments. We show that the Sam closure phases, interpreted as the signature of a candidate companion, may actually trace the asymmetry generated by forward scattering by dust grains in the upper layers of the outer disk. These observations help constrain the inclination and position angle of the outer disk. The presence of matter inside the gap is difficult to assess with present-day observations. Our model suggests the outer disk contaminates the interferometric signature of any potential companion that could be responsible for the gap opening, and such a companion still has to be unambiguously detected. We stress the difficulty to observe point sources in bright massive disks, and the consequent need to account for disk asymmetries (e.g. anisotropic scattering) in model-dependent search for companions.

Read more (998kb, PDF)

Blobrana · L

Title: 2M1155-79 (= T Cha B): A Low-mass, Wide-separation Companion to the Nearby, "Old" T Tauri Star T Cha
Authors: Joel H. Kastner (1), E. Thompson (1,2), R. Montez Jr. (1), S. J. Murphy (3), M. S. Bessell (3), G. G. Sacco (1,4) ((1) Center for Imaging Science and Laboratory for Multiwavelength Astrophysics, Rochester Institute of Technology, Rochester, NY, USA, (2) West Irondequoit High School, Rochester, NY, USA, (3) RSAA, Mount Stromlo Observatory, The Australian National University, (4) INAF-Osservatorio Astrofisico di Arcetri, Florence, Italy)

The early-K star T Cha, a member of the relatively nearby (D ~ 100 pc) epsilon Cha Association, is a relatively "old" (age ~7 Myr) T Tauri star that is still sporadically accreting from an orbiting disk whose inner regions are evidently now being cleared by a close, substellar companion. We report the identification, via analysis of proper motions, serendipitous X-ray imaging spectroscopy, and followup optical spectroscopy, of a new member of the epsilon Cha Association that is very likely a low-mass companion to T Cha at a projected separation of ~38 kAU. The combined X-ray and optical spectroscopy data indicate that the companion, T Cha B (= 2M1155-79), is a weak-lined T Tauri star (wTTS) of spectral type M3 and age ~<10 Myr. The serendipitous X-ray (XMM-Newton) observation of T Cha B, which targeted T Cha, also yields serendipitous detections of two background wTTS in the Chamaeleon cloud complex, including one newly discovered, low-mass member of the Cha cloud pre-MS population. T Cha becomes the third prominent example of a nearby, "old" yet still actively accreting, K-type pre-MS star/disk system (the others being TW Hya and V4046 Sgr) to feature a low-mass companion at very large (12-40 kAU) separation, suggesting that such wide-separation companions may affect the conditions and timescales for planet formation around solar-mass stars.

Read more (427kb, PDF)

Blobrana · L

Title: Herschel Observations of the T Cha Transition Disk: Constraining the Outer Disk Properties
Authors: Lucas A. Cieza, Johan Olofsson, Paul M. Harvey, Christophe Pinte, Bruno Merin, Jean-Charles Augereau, Neal J. Evans II, Joan Najita, Thomas Henning, Francois Menard

T Cha is a nearby (d = 100 pc) transition disk known to have an optically thin gap separating optically thick inner and outer disk components. Huelamo et al. (2011) recently reported the presence of a low-mass object candidate within the gap of the T Cha disk, giving credence to the suspected planetary origin of this gap. Here we present the Herschel photometry (70, 160, 250, 350, and 500 micron) of T Cha from the "Dust, Ice, and Gas in Time" (DIGIT) Key Program, which bridges the wavelength range between existing Spitzer and millimetre data and provide important constraints on the outer disk properties of this extraordinary system. We model the entire optical to millimetre wavelength spectral energy distribution (SED) of T Cha (19 data points between 0.36 and 3300 micron without any major gaps in wavelength coverage). T Cha shows a steep spectral slope in the far-IR, which we find clearly favours models with outer disks containing little or no dust beyond 40 AU. The full SED can be modelled equally well with either an outer disk that is very compact (only a few AU wide) or a much larger one that has a very steep surface density profile. That is, T Cha's outer disk seems to be either very small or very tenuous. Both scenarios suggest a highly unusual outer disk and have important but different implications for the nature of T Cha. Spatially resolved images are needed to distinguish between the two scenarios.

Read more (409kb, PDF)

Blobrana · L

Scientists see possible birth of a planet

A faint object seen orbiting a young star may be the first planet observed during its formation.
An international team of researchers, including two University of Sydney astronomers, used high-powered telescopes in Chile to capture images of a celestial body carving an orbit around the star T Cha.
An astronomer and contributing researcher, Peter Tuthill, said uncovering a planet in the midst of formation was the "holy grail" of astronomy because it would allow researchers to study how planets developed.
Read more

Blobrana · L

Blobrana · L

Title: A companion candidate in the gap of the T Cha transitional disk
Authors: N. Huelamo, S. Lacour, P. Tuthill, M. Ireland, A.Kraus, G. Chauvin

T Cha is a young star surrounded by a cold disk. The presence of a gap within its disk, inferred from fitting to the spectral energy distribution, has suggested on-going planetary formation. We observed T Cha in L' and K_s with NAOS-CONICA, the adaptive optics system at the VLT, using sparse aperture masking. We detected a source in the L' data at a separation of 62±7 mas, position angle of 78±1 degrees, and a contrast of delta L' = 5.1±0.2 mag. The object is not detected in the Ks band data, which show a 3-sigma contrast limit of 5.2 mag at the position of the detected L' source. For a distance of 108 pc, the detected companion candidate is located at 6.7 AU from the primary, well within the disk gap. If T Cha and the companion candidate are bound, the comparison of the L' and Ks photometry with evolutionary tracks shows that the photometry is inconsistent with any unextincted photosphere at the age and distance of T Cha. The detected object shows a very red Ks-L' colour for which a possible explanation would be a significant amount of dust around it. This would imply that the companion candidate is young, which would strengthen the case for a physical companion, and moreover that the object would be in the substellar regime, according to the Ks upper limit. Another exciting possibility would be that this companion is a recently formed planet within the disk. Additional observations are mandatory to confirm that the object is bound and to properly characterise it.

Read more (721kb, PDF)

Blobrana · L

Planet Formation in Action?

Using ESO's Very Large Telescope an international team of astronomers has been able to study the short-lived disc of material around a young star that is in the early stages of making a planetary system. For the first time a smaller companion could be detected that may be the cause of the large gap found in the disc. Future observations will determine whether this companion is a planet or a brown dwarf.
Planets form from the discs of material around young stars, but the transition from dust disc to planetary system is rapid and few objects are caught during this phase. One such object is T Chamaeleontis (T Cha), a faint star in the small southern constellation of Chamaeleon that is comparable to the Sun, but very near the beginning of its life. T Cha lies about 350 light-years from the Earth and is only about seven million years old. Up to now no forming planets have been found in these transitional discs, although planets in more mature discs have been seen before.
Read more

Blobrana · L

Title: Variability of the transitional T Tauri star T Chamaeleontis
Authors: E. Schisano, E. Covino, J.M. Alcala', M. Esposito, D. Gandolfi, E.W. Guenther
(Version v2)

We characterise the physical properties of T Chamaeleontis, a transitional T Tauri star showing UX Ori-type variability, and of its associated disc, and probe possible effects of disc clearing processes. Different spectral diagnostics are examined, based on a rich collection of optical high- and low-resolution spectra. We determine radial and projected rotational velocities, and measure equivalent widths of the Li I (6708 A) line and of the most prominent emission lines (e.g. Ha, Hb and [OI] 6300A); we analyse shape changes of photospheric lines via bisector-method, while variability in Ha and Hb is inspected through line-profile correlation matrices. The strength of the Ha and Hb emission is highly variable and well correlated with that of the [OI] lines, as well as with Av variations of over three magnitudes. Variations up to nearly 10 km/s in the radial velocity of the star are measured on analogous time-scale, but with no apparent periodicity. SED modelling confirms the existence of a gap in the disc. Variable circumstellar extinction is pointed out as responsible for the conspicuous variations observed in the stellar continuum flux and for concomitant changes in the emission features by contrast effect. Clumpy structures, incorporating large dust grains and orbiting the star within a few tenths of AU, obscure episodically the star and, eventually, part of the inner circumstellar zone, while the bulk of the hydrogen lines emitting zone and outer low-density wind region traced by the [OI] remain unaffected. Coherently with this scenario, the detected radial velocity changes are also explainable in terms of clumpy materials transiting and partially obscuring the star.

Read more (2938kb, PDF)