Discovery of “isolated” co-moving T Tauri stars in Cepheus

Abstract

Context. During the course of a large spectroscopic survey of X-ray active late-type stars in the solar neighbourhood, we discovered four lithium-rich stars packed within just a few degrees on the sky. Although located in a sky area rich in CO molecular regions and dark clouds, the Cepheus-Cassiopeia complex, these very young stars are projected several degrees away from clouds in front of an area void of interstellar matter. As such, they are very good "isolated" T Tauri star candidates. Aims. We present optical observations of these stars conducted with 1-2 m class telescopes. We acquired high-resolution optical spectra as well as photometric data allowing us to investigate in detail their nature and physical parameters with the aim of testing the "runaway" and "in-situ" formation scenarios. Their kinematical properties are also analyzed to investigate their possible connection to already known stellar kinematic groups. Methods. We use the cross-correlation technique and other tools developed by us to derive accurate radial and rotational velocities and perform an automatic spectral classification. The spectral subtraction technique is used to infer chromospheric activity level in the Hα line core and clean the spectra of photospheric lines before measuring the equivalent width of the lithium absorption line. Results. Both physical (lithium content, chromospheric, and coronal activities) and kinematical indicators show that all stars are very young, with ages probably in the range 10-30 Myr. In particular, the spectral energy distribution of TYC 4496-780-1 displays a strong near-and far-infrared excess, typical of T Tauri stars still surrounded by an accretion disc. They also share the same Galactic motion, proving that they form a homogeneous moving group of stars with the same origin. Conclusions. The most plausible explanation of how these "isolated" T Tauri stars formed is the "in-situ" model, although accurate distances are needed to clarify their connection with the Cepheus-Cassiopeia complex. The discovery of this loose association of "isolated" T Tauri stars can help to shed light on atypical formation processes of stars and planets in low-mass clouds.

The Hercules-Lyra association revisited

“…The gyrochronology suggests ∼50 Myr. The celestial location co-incides with the Cepheus-Cassiopeia complex and a group of four co-moving isolated T Tauri stars identified by Guillout et al (2010). They argue, however, that the proper motion and the distances rule out a common origin with V368 Cep and its companions.…”

Reliable probabilistic determination of membership in stellar kinematic groups in the young disk

“…Only the UMa stream is clearly separated from the others in the (U, V) plane. It is not unusual that one source may be reported as a candidate of several groups of comoving stars (see, Guillout et al 2009Guillout et al , 2010. In such cases, the stellar properties (and a coherent sky position for the candidate to MGs) help recognize the best SKG match.…”

“…From the analysis of the RasTyc sample (Guillout et al 2009;Frasca et al, in prep. ), four comoving T Tauri stars were recently discovered in a sky region of few degrees toward the CO Cepheus void (Guillout et al 2010). Klutsch et al (2011b) extended the member list by the identification of additional young stars in the sky area that surrounds this stellar group.…”

A spectroscopic survey of the youngest field stars in the solar neighborhood

“…They form a homogeneous moving group, with a likely age of 10-30 Myr. We published a preview of the results for these four stars in Guillout et al (2010), but we include them in the present paper where additional spectra have been also analyzed.…”