The accretion rates and mechanisms of Herbig Ae/Be stars

Abstract

This work presents a spectroscopic study of 163 Herbig Ae/Be stars. Amongst these, we present new data for 30 objects. Stellar parameters such as temperature, reddening, mass, luminosity and age are homogeneously determined. Mass accretion rates are determined from Hα emission line measurements. Our data is complemented with the X-Shooter sample from previous studies and we update results using Gaia DR2 parallaxes giving a total of 78 objects with homogeneously determined stellar parameters and mass accretion rates. In addition, mass accretion rates of an additional 85 HAeBes are determined. We confirm previous findings that the mass accretion rate increases as a function of stellar mass, and the existence of a different slope for lower and higher mass stars respectively. The mass where the slope changes is determined to be 3.98 +1.37 −0.94 M . We discuss this break in the context of different modes of disk accretion for low-and high mass stars. Because of their similarities with T Tauri stars, we identify the accretion mechanism for the late-type Herbig stars with the Magnetospheric Accretion. The possibilities for the earlier-type stars are still open, we suggest the Boundary Layer accretion model may be a viable alternative. Finally, we investigated the mass accretion -age relationship. Even using the superior Gaia based data, it proved hard to select a large enough sub-sample to remove the mass dependency in this relationship. Yet, it would appear that the mass accretion does decline with age as expected from basic theoretical considerations.

On the Mass Accretion Rates of Herbig Ae/Be Stars. Magnetospheric Accretion or Boundary Layer?

“…In contrast, the L acc -L * trend followed by HAes when their accretion luminosities are inferred from MA is overplotted in green. This is equal within errorbars to the one followed by TTs and has a smaller slope ∼1 (see the bottom right panel of Figure 2 and [65,66] for details). [48] were used to infer the L acc values, as well as their data for M * and R * to derive the final MA-based values forṀ acc = L acc R * /GM * .…”

“…The mismatch between MA and BL estimates is in last term due to differences between the underlying assumptions concerning energy transformation under both approaches (e.g., [66]). In MA the gravitational energy is carried through the accretion funnels and released at hot spots in the stellar surface, whereas in BL the kinetic energy of the rotating disk heats the BL when material is drastically decelerated in this region.…”

“…This uncertainty could in principle be reduced by averaging the results obtained from different emission lines [41]. The most recent L acc -L line empirical expressions for HAeBes including dozens of emission lines can be found in Fairlamb et al (2017) [64], and accretion rates inferred from this method (in particular, from the correlation with L Hα ) can also be found in the literature for hundreds of HAeBes [66,68].…”

Catalogue of new Herbig Ae/Be and classical Be stars

“…6, extinctions and effective temperatures are present in Vioque et al 2018 and references therein). Combining the Hα EWs of Vioque et al (2018) and Wichittanakom et al (2020) with the few sources present in IPHAS or VPHAS+ gave us r − Hα colour for 215 HAeBes. This is why neither r nor Hα passbands are combined with the rest in Sect.…”

“…Ababakr et al 2017), and spectro-photometric observations (e.g. Mendigutía et al 2011;Fairlamb et al 2015;Wichittanakom et al 2020) have shown that the lower mass Herbig Ae stars show accretion signatures consistent with T-Tauri stars, whereas Herbig Be stars appear to be inconsistent with magnetospheric accretion. A large caveat in these studies is that they do not include the less evolved sources in highmass PMS tracks (most Herbig Be stars observed to date are very close to the main sequence, Vioque et al 2018), which are obviously of paramount importance for understanding highmass accretion.…”