Activity on young stars.

Details Activity on young stars. Activity on young stars.

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...301...89g&link_type=abstract

Astronomy and Astrophysics, v.301, p.89

Astronomy and Astrophysics

Astrophysics

24

Stars: Activity, Stars: Flare, Stars: Pre-Main-Sequence, Stars: Variable

Scientific paper

Simultaneous photometry and spectroscopy were made of 6 young stars during two observing periods mainly to study short-term variability on time-scales from minutes to a few hours. The material includes two classical T Tauri stars (CTTS): SY Ori and VW Cha; three T Tauri stars with weak emission line spectra (WTTS): San 1, SZ Cha and ADA 481 and one post-T Tauri candidate: HD 70309B. Both UBV and Stroemgren photometry was made. In the visible spectral region we resolved rapid fluctuations - events - with total amplitudes of about 5% (0.05 magnitudes). In the ultraviolet, the corresponding limit of detection was usually <=10%. On the basis of totally about 100 hours of monitoring we conclude that the normal state of these stars is that they are completely constant in brightness or that they vary only slowly with small amplitudes over several hours. Only a few percent of the time, on the average, is a given star caught at brightness changes >=0.2mag. during one night. No event reached a total amplitude of >=0.3mag. VW Cha is the most active star, but no events were seen on SY Ori and HD 70309B. This confirms earlier indications that powerful "flaring" on T Tauri stars is not frequent. We make a detailed study of all events and find two types of slow events, usually with d(U or u)/dt<=0.1mag/hour. One is caused by changes in the continuous emission (the veiling) superimposed on the stellar photospheric spectrum and operates mainly on VW Cha. These events have nothing to do with stellar surface flares of the type observed on flare stars and we suggest that they originate from inhomogeneous mass accretion from a circumstellar disk to the stellar surface. The time-scales support models with magnetically controlled accretion along the stellar dipole field to rings or spots at the stellar surface. The other type of event appears to originate from relatively rapid changes in the opacity of circumstellar dust in the line-of-sight to the star. This effect dominates on SZ Cha, a WTTS surrounded by a substantial dust reservoir. Also for the rapid events we distinguish two types. On two WTTS we detected a few flare-like events produced by a sudden increase in emission in the Balmer continuum and the Balmer lines and no detectable change of the continuum long-ward of the Balmer jump. With only UBV photometry the Balmer flares could erronously been interpreted as very hot blackbody radiators. We suggest that these events are genuine surface flares with total energies of 10^33^ to 10^34^erg, and discuss the implication of energy supply. On ADA 481 we detected 2 flare-like events in white light. If these are due to the ignition of a source of blackbody radiation, the inferred temperature of the flare is low compared to what is normally observed for flare stars. Even though the events are rare and have small total amplitudes in UV, they are extremely powerful, with the same total energies as the largest flares seen on flare stars. The flare stars may show much larger changes in UV, but the difference comes from the lower contrast of the flares on the TTS. If all TTS have surface magnetic activity similar to the flare stars, only the radii being larger, then we conclude that the frequency distribution of the flare-like events on WTTS are similar to flare stars in the field, but much higher than for the dwarfs in the Pleiades. No flare-like event was seen on the CTTS and we discuss possible implications. For the long-term changes (over days) we conclude that very dark spots on the rotating surfaces of SY Ori and San 1 dominates, while VW Cha varies because of variable veiling, but with an uncertain period. For SZ Cha variable circumstellar extinction operates, also in phase with the hydrogen line absorption. The situation for ADA 481 is still unclear. HD 70309B did not vary.

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