17th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun

Type: Oral

Topic: Evolved Stars and Hot-Cool Star Connection

Abstract
Planetary systems are eventually formed from the evolution of gas and dust disks that surround young pre-main sequence (PMS) stars. Given the difficulties to make direct gas detections in protoplanetary disks, an indirect gas tracer such as the accretion rate is a key parameter that is crucial to estimate. Nowadays, there is consensus on the way that disk matter is transferred onto the central objects, at least for cool, low-mass PMS stars (T $<$ 7000 $K$, M $<$ 1 M$_{\odot}$, i.e. the so-called T Tauri stars). Magnetospheric accretion models are successfully applied to these objects, allowing us to estimate their accretion rates from different tracers. Recently (Mendigut\'\i{}a et al. 2011, A\&A, 535, A99), we have estimated magnetospheric accretion rates also for 38 hotter, intermediate-mass PMS stars (7000 $<$ T($K$) $<$ 13000; 1 $<$ M(M$_{\odot}$) $<$ 6; Herbig Ae/Bes). Accretion rates for both T Tauri and Herbig Ae/Be objects have been related to several accretion tracers, as well as to stellar and disk parameters derived from their spectral energy distributions. I will discuss the results regarding the comparison between the accretion-related properties for both cool and hot regimes, which are useful to look for analogies/differences in the physics of the star-disk interaction, and in the physical mechanisms driving disk dissipation.\\ In summary, we have found that several optical and IR line luminosities used for low-mass objects are also valid to estimate typical accretion rates for intermediate-mass stars under similar empirical expressions. In contrast, the H$\alpha$ width at 10$\%$ of peak intensity is commonly used as an accretion tracer for T Tauris, but is not reliable to estimate accretion rates for Herbig Ae/Bes. This can be explained as a consequence of the different stellar rotation rates that characterize both types of stars. In addition, we find similarities when the accretion rate is related to the near-IR colours and disk masses, suggesting that viscous accretion disk models are able to explain these trends for both T Tauri and Herbig Ae/Be stars. We also find two major differences between cool and hot PMS objects. First, the inner gas dissipation timescale, as estimated by relating the accretion rates and the stellar ages, is slightly faster for Herbig Ae/Be stars. This could have implications on the physical mechanism able to form planets around objects more massive than the Sun. Second, the relative position of Herbig Ae/Bes with disks showing signs of inner dust clearing in the accretion rate--disk mass plane contrasts with that of T Tauri stars with transitional disks, when both samples are compared with ''classical'', non-evolved disks. I will discuss how this latter difference could be pointing to a different physical mechanism driving disk evolution, depending on the stellar regime considered (Mendigut\'\i{}a et al. 2012, submitted to A\&A).