Tracing Accretion onto Herbig Ae/Be Stars Using the Brγ Line

Abstract

Accretion plays an important role in protoplanetary disk evolution, and it is thought that the accretion mechanism changes between low- and high-mass stars. Here we characterize accretion in intermediate-mass, pre-main-sequence Herbig Ae/Be (HAeBe) stars to search for correlations between accretion and system properties. We present new high-resolution, near-infrared spectra from the Immersion GRating INfrared Spectrograph for 102 HAeBes and analyze the accretion-tracing Brγ line at 2.166 μm. We also include the samples of Fairlamb et al. and Donehew & Brittain, for a total of 155 targets. We find a positive correlation between the Brγ and stellar luminosity, with a change in the slope between the Herbig Aes and Bes. We use L Brγ to determine the accretion luminosity and rate. We find that the accretion luminosity and rate depend on stellar mass and age; however, the trend disappears when normalizing the accretion luminosity by the stellar luminosity. We classify the objects into flared (group I) or flat (group II) disks and find that there is no trend with accretion luminosity or rate, indicating that the disk dust structure is not impacting accretion. We test for Brγ variability in objects that are common to our sample and previous studies. We find that the Brγ line equivalent width is largely consistent between the literature observations and those that we present here, except in a few cases where we may be seeing changes in the accretion rate.

Identification and Spectroscopic Characterization of 128 New Herbig Stars*

“…We suspect that the small difference between both groups is caused by the lower resolution of our observations, which moved many P-Cygni and double-peak profiles to the "single-peaked" group. We point out that similar percentages were found for the Brγ line by Grant et al (2022) in known Herbig stars.…”

“…The break between both groups was set at -+ 3.98 0.94 1.37 M e . This accretion break and similar accretion gradients to those found in Wichittanakom et al (2020) have also been identified by Grant et al (2022), using a similar sample and the Brγ line as the accretion tracer.…”

“…In this section, we interpret the break in accretion gradient at 3-4 M e identified in Wichittanakom et al (2020) and Grant et al (2022), that we extend to the new Herbig stars in Section 7 and constrain to -+ 3.87 0.96 0.38 M e . The most accepted interpretation for this change in accretion gradient is that lowmass objects are subjected to magnetospheric accretion (see Bouvier et al 2007), whereas a fraction of the more massive objects are accreting through a different mechanism; possibly the boundary-layer accretion mechanism (Mendigutía 2020).…”

“…Whereas the formation of low-mass stars is widely accepted to be due to magnetically controlled accretion (Bouvier et al 2007), higher mass stars are generally non-magnetic, and it would be expected that this scenario does not apply to them (see Mendigutía 2020). Recent studies (e.g., Wichittanakom et al 2020;Grant et al 2022) have provided evidence for the change in accretion mechanism happening at around 4 M e .…”

Recent Progress in Finding Binary Systems with the B[e] Phenomenon

“…There is still a controversy about the evolutionary classification of all of them. In some studies they are considered pre-main-sequence Herbig Be stars (e.g., [75][76][77] to name a few recent ones), but there has not been an unambiguous conclusion as to their extreme youth. Neither of these objects belongs to a star-forming region or has a strong far-IR excess from distant CS dust radiation that is similar to those of true pre-main-sequence stars.…”