Inferring the Veiling Spectrum of LkCa 15

Abstract goes here.

Introduction

The “classical” T Tauri phase is in part defined by the spectroscopic evidence for accretion from a disk onto the host young star. These accretion signatures are manifested as a sequence of bright, broad emission lines with complex velocity structure, produced in magnetospheric funnel flows (e.g., Muzerolle et al. 1998) and a corresponding wind (e.g., Dupree et al. 2012), as well as in a pronounced “veiling” continuum that partially fills in the standard photospheric absorption lines, generated by the shock of accreting material impacting the stellar surface (e.g., Hartigan et al. 1991; Calvet & Gullbring 1998). The so-called transition disk hosts are a particularly exciting sub-class of classical T Tauri stars, exhibiting substantial clearing in the inner regions of their disks despite evidence for relatively strong accretion (e.g., Andrews et al. 2011; Espaillat et al. 2012). It is unclear how that disk material is being accelerated across these cleared regions; some have speculated that streamers of material are launched by dynamical interactions between the outer disk and unseen (possibly planetary) companions near the disk edge (e.g., Zhu et al. 2011; Dodson-Robinson & Salyk 2011). In that scenario, constraints on time variability in these accretion signatures could teach us about the flow of material near the putative planetary companions.

The veiling spectrum itself is also worthy of more general study because it confounds attempts to understand the fundamental properties of accreting T Tauri star photospheres. Traditionally, astronomers measure the veiling continuum by comparing the accreting T Tauri star target with a template young star of similar spectral type but negligible accretion rate (a “weak-lined” T Tauri star). Optical veiling excesses inferred in this way typically lie in the \(r\sim 0.1\)-1 range (10-100% relative to the photosphere), with a roughly flat spectrum from the Balmer jump to \(\sim\)1 \(\mu\)m (e.g., Herczeg & Hillenbrand 2014). However, different groups routinely find discrepant excess levels in the same target (e.g., see Gullbring et al. 2000). Interestingly, some have argued that this represents intrinsic accretion variability (e.g., Bouvier et al. 2007; Kurosawa & Romanova 2013). However, there are two potential sources of systematic uncertainty in such measurements that are usually not considered. The first is a static issue imposed by slight mismatches in the target and template properties: in essence, a similar spectral classification pairing in no way guarantees that any differences in effective temperature, surface gravity, or other parameters (e.g., projected rotation velocities) between the target and template stars will not bias the veiling estimate. The second is a temporal issue, caused by the known (and incoherent) variability of both the template and the target, primarily due to starspots (e.g., Bouvier et al. 1993; Herbst et al. 1994).

We seek to link both of these larger issues by observing a trimester-long time series of high resolution optical spectra for a nearby accreting transition disk host (LkCa 15) and associated weak-line T Tauri star template (LkCa 14) with a 2–3 day cadence. With these data, we will infer constraints on key stellar parameters (\(T_{\rm eff}\), \(\log{g}\), \(v\sin{i}\), etc.) and optical veiling measurements at \(\sim\)25 distinct epochs in this target–template pairing, providing a robust examination of potential evidence for accretion rate variability in the LkCa 15 system, and its associated systematic uncertainty in the presence of potential pairing mismatch and stellar activity. Moreover, such data will provide a relatively controlled experiment for us to develop a phenomenological methodology for forward-modeling both starspots and veiling excesses that can ultimately be applied to observations of other systems.

The dimensionsless quantity defining the degree of veileng is defined by McClure et al. (2013) as

\begin{equation} r(\lambda)=\frac{f_{V}(\lambda)}{f_{C}(\lambda)}\\ \end{equation}

where \(f_{V}\) is the veiling spectrum and \(f_{C}\) is the inferred continuum of the star, which both have units of flux.

Description of the LkCa14 and LkCa15 systems. Brief mention of planet.