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.

Data

XX spectra were acquired with the TRES echelle spectrograph on the 1.5m telescope at Mt. Hopkins.

Each spectrum is a 51-order echelle spectrum, where the echelle order is denoted by \(m\). Each order contains the same number of pixels, \(N=2298\).

The spectra are corrected for the blaze function, although calibration issues remain. Typically these issues are addressed through continuum normalization. In our case, we aim to infer the normalization through Chebyshev polynomials.

Star JD [day] Exposure (s) peak S/N
LkCa 14 2456931.0074 1500.0 44.9
LkCa 14 2456931.9980 1500.0 50.0
LkCa 14 2456933.9592 1800.0 31.3
LkCa 14 2456935.9921 1500.0 50.6
LkCa 14 2456941.9253 1500.0 43.2
LkCa 14 2456942.9103 1500.0 52.3
LkCa 14 2456943.9467 1500.0 51.3
LkCa 14 2456944.9157 1500.0 42.1
LkCa 14 2456958.8798 1500.0 50.6
LkCa 14 2456959.9483 1500.0 45.0
LkCa 14 2456962.8816 1500.0 50.7
LkCa 14 2456968.8212 3600.0 44.4
LkCa 14 2456974.9704 1650.0 41.2
LkCa 14 2456989.9242 1650.0 46.2
LkCa 14 2456991.8762 3000.0 61.4
LkCa 14 2456992.7862 1590.0 44.7
LkCa 14 2456992.9937 1560.0 39.2
LkCa 14 2456996.7224 3000.0 59.1
LkCa 14 2457002.8033 1500.0 47.5
LkCa 14 2457003.6691 2100.0 51.6
LkCa 15 2456578.9257 3600.0 64.8
LkCa 15 2456606.8669 3600.0 53.3
LkCa 15 2456650.7815 3600.0 51.7
LkCa 15 2456930.9846 1800.0 33.7
LkCa 15 2456931.9770 1800.0 43.4
LkCa 15 2456933.9838 2160.0 33.7
LkCa 15 2456935.9714 1800.0 43.1
LkCa 15 2456941.9477 1800.0 41.5
LkCa 15 2456942.9329 1800.0 47.9
LkCa 15 2456943.9682 1800.0 48.6
LkCa 15 2456944.9367 1800.0 46.4
LkCa 15 2456958.9018 1800.0 43.9
LkCa 15 2456959.9693 1800.0 37.8
LkCa 15 2456962.9188 1800.0 27.8
LkCa 15 2456968.8665 3600.0 37.7
LkCa 15 2456974.9932 1950.0 44.6
LkCa 15 2456989.9478 1890.0 29.0
LkCa 15 2456991.9340 3600.0 50.0
LkCa 15 2456992.7616 2190.0 132.5
LkCa 15 2456992.9717 1890.0 38.8
LkCa 15 2456996.7638 3600.0 58.9
LkCa 15 2457002.8258 1800.0 51.4
LkCa 15 2457003.7716 4500.0 64.2

Note. – TRES observations.