EVOLUTION OF THE DUST/GAS ENVIRONMENT AROUND HERBIG Ae/Be STARS

Using the KOSMA 3 m telescope, 54 Herbig Ae/Be (HAe/Be) stars were surveyed in CO and (13)CO emission lines. The properties of the stars and their circumstellar environments were studied by fitting spectral energy distributions (SEDs). The mean line width of (13)CO (2-1) lines of this sample is 1.87 km s(-1). The average column density of H(2) is found to be 4.9 x 10(21) cm(-2) for stars younger than 10(6) yr, while this drops to 2.5 x 10(21) cm(-2) for those older than 10(6) yr. No significant difference is found among the SEDs of HAe and HBe stars of the same age. Infrared excess decreases with age, envelope masses and envelope accretion rates decease with age after 10(5) yr, the average disk mass of the sample is 3.3 x 10(-2) M(circle dot), the disk accretion rate decreases more slowly than the envelope accretion rate, and a strong correlation between the CO line intensity and the envelope mass is found.