The UV Perspective of Low-Mass Star Formation

Abstract

The formation of low-mass (M 2 M ) stars in molecular clouds involves accretion disks and jets, which are of broad astrophysical interest. Accreting stars represent the closest examples of these phenomena. Star and planet formation are also intimately connected, setting the starting point for planetary systems like our own. The ultraviolet (UV) spectral range is particularly suited to study star formation, because virtually all relevant processes radiate at temperatures associated with UV emission processes or have strong observational signatures in the UV. In this review, we describe how UV observations provide unique diagnostics for the accretion process, the physical properties of the protoplanetary disk, and jets and outflows.CTTSs were initially identified as a new class of objects due their optical variability. Very early on, it was realized that they also show signs of enhanced chromospheric activity, i.e., "emission lines resembling those of the solar chromosphere" [2]. These emission lines are superposed on a photospheric spectrum similar to main sequence stars of late spectral type. Initially, the source of emission in excess of a normal photosphere was speculated to be circumstellar or chromospheric in origin [3].The discovery that CTTSs are above and to the right of the main sequence in an HR-diagram demonstrated in conjunction with theoretical work [4], that these objects must be young and, thus, the progenitors of the large main sequence population [5,6]. This notion is corroborated by the presence of strong Li absorption, which requires ∼ 100× the Li abundance of the Sun [e.g. 7]. Because Li is depleted quickly in stellar interiors, the amount of surface lithium decreases with time in convective stars. Therefore, strong Li absorption can only be found in young stars and thus CTTS must be young [e.g. review by 8].Meanwhile the number of peculiarities found in CTTSs grew, but their established youth alone was insufficient to explain those peculiarities and the "mysteries" of CTTSs remained. Many features, like the origin of the "chromospheric emission" remained unexplained, until the early 1980s when a large variety of models were proposed to explain features of CTTSs including envelopes, outflows, infall, and dust disks.Specifically, the "ultraviolet excess" (measured around 3700 Å) and the "blue continuum", which go along with a weakening of photosperic absorption lines due to veiling [9,10], were shown to correlate with the strength of Hα [11,12]. Such a correlation indicates a common origin of both features as observed for chromospheric emission on the Sun. Thus, it was natural to ascribe the excess continuum emission at short wavelengths to a Balmer continuum, i.e., emission following the capture of a free electron by an ionized hydrogen atom. While that finding pinpoints neither the physical origin nor location of the emitting material, it paved the way for explanations involving a suitably tuned chromosphere for producing the observed (Balmer & Ca II H+K) line fluxes in CTTSs. Quantitative mod...

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

“…Although some controversy still remains concerning the "disk-locking" view (see e.g., the related discussion in [31]), the main lines of evidence summarized above are supported by many independent works that reached a consensus about the validity of MA against BL. A similar discussion about MA in CTTs as observed at short UV and X-ray wavelengths can be found in the review by Schneider et al (2020) [32] for this same special issue of the journal.…”

Understanding Accretion Variability through TESS Observations of Taurus

“…Accretion also provides a method of probing the inner regions of the disk, which traditional observational techniques lack the ability to resolve. For two recent reviews on accretion onto CTTS, see Hartmann et al (2016) and Schneider et al (2020).…”

Monitoring accretion rate variability in the Orion Nebula Cluster with the Wendelstein Wide Field Imager

“…The accretion rate variability manifests itself mainly in the optical and near-UV bands (e.g., Venuti et al 2014Venuti et al , 2015Robinson & Espaillat 2019;Schneider et al 2020). The observed brightness variations stem from a multitude of mechanisms, including accretion bursts (e.g., Venuti et al 2014), inner disk warps (e.g., Frasca et al 2020), accretion columns creating hot spots (e.g., Cody et al 2014) moving in and out of view while the YSO rotates (e.g., Cody & Hillenbrand 2018;Schneider et al 2020) and phases of stable or unstable accretion regimes (e.g., Sousa et al 2016).…”

“…The UV excess is a rather direct tracer of the accretion rate (e.g., Schneider et al 2020). In this study, we are interested in characterizing the accretion rate variability through the UV excess of a particular sample of young stars in the ONC in order to analyze how this variability influences our statistical analysis of the relation between X-ray luminosities and accretion rates on timescales of weeks to a few years.…”