Search for new T Tauri stars in the Cepheus-Cassiopeia region

Abstract

Abstract. The Cepheus-Cassiopeia star-forming region has been searched for new T Tauri stars (TTSs) based on the ROSAT all sky survey (RASS). Optical spectroscopic observations were carried out toward 45 GSC stellar counterparts of RASS sources looking for the Li absorption line (6708 Å), a sign of youth. The detection of this line resulted in finding 11 to 16 new TTSs in this region. Using follow-up optical photometric observations and evolutionary models these Li-rich stars are revealed to be young low-mass stars. The most of the young stars are separated from the 13 CO molecular clouds by ∼10 pc, significantly more than in Chamaeleon. A group of the new TTSs are isolated from the CO clouds and distributed inside the previously determined CO void. Possible formation scenarios including the interaction with the supernova shock for the isolated TTSs are discussed.Key words. ISM: clouds -ISM: molecules -stars: formation -stars: late-type -stars: pre-main sequence -X-rays: stars Introduction: The Cepheus-Cassiopeia star-forming regionLarge-scale X-ray surveys are a powerful method to search for T Tauri stars (TTSs) that are widely spread over starforming regions (SFRs). Spectroscopic follow-up observations towards the X-ray sources found by the ROSAT X-ray satellite have been carried out in nearby SFRs such as Taurus ), Chamaeleon (Alcalá et al. 1995), and Lupus (Wichmann et al. 1997, most of which are thought to belong to the Gould belt. These surveys found a large number of T Tauri stars (see Neuhäuser 1997 for a review). One of the interesting results of the X-ray selected TTS surveys is that a large number of TTSs have been found far away from the molecular clouds. Because of their youth ( < ∼ a few ×10 7 yr), they are supposed to be distributed within ∼10 pc of their birthplaces. Nonetheless, many so-called "isolated TTSs", separated from the molecular clouds by more than a few tens of pc, are found, for example, south of the Taurus and η Cha clusters (Mamajek et al. 1999), even though many of those south of Taurus were later reported to be active binaries (Torres et al. 2002). Recent studies suggest that such isolation is likely caused by in situ star formation in small clouds and their subsequent rapid dissipation (e.g., Mizuno et al. 1998;Tachihara et al. 2001).Cepheus-Cassiopeia (Cep-Cas) is a nearby SFR located at l ∼ 100 • -140 • and b ∼ 5 • -25 • . The region has been surveyed extensively for molecular clouds in 12 CO by Grenier et al. (1989) who found a total molecular mass of ∼10 5 M . They reported that the cloud distribution shows a CO void which contains an excess of soft X-rays and radio continuum, suggesting a hot bubble created by a supernova. A distance estimation to several dark clouds have been carried out by measuring the colour excess or utilizing star counts (Kun & Prusti 1993;Kun et al. 1994;Kun 1998;Obayashi et al. 1998). Yonekura et al. (1997) surveyed the region in 13 CO and detected 188 distinct molecular clouds. They obtained different radial velocities suggesting that layers at d...

Variability of young stars: Determination of rotational periods of weak‐line T Tauri stars in the Cepheus‐Cassiopeia star‐forming region

“…Fig.16 shows the histogramm of all observed periods. Tachihara et al (2005) To determine the periods the best algorithm seems to be the Lomb-Scargle, although stringlength works without fitting sine waves. In case of short periodic variations it reveals its shortcoming by determining multiples of the correct period.…”

“…The circles (black, grey, without filling) mark the observed objects with detection of strong Li, weak Li and no Li, respectively. For further information see Tachihara et al (2005). Out of this sample ten objects were choosen.…”

“…To create the artificial CS only eight objects out of 576 detected were used. This small number of CS has two reasons: No star within the field seems to be very constant, and depending on the selection of the CS this object shows Tachihara et al (2005) different light curves in V , R and I. Hence, only those stars were used which are weighted higher than 10 −2 within every filter.…”

“…Spectroscopic observations started in 2000. With these observations Tachihara et al (2005) tried to find possible TTS. Out of 46 ROSAT selected X-ray sources 16 objects showed strong Li abundance and are a few Myr young.…”

“…The galactic position of the 46 targets is given in Fig.1. The ten objects observed and presented here were selected from Tachihara et al (2005).…”