Placing the Spotted T Tauri Star LkCa 4 on an HR Diagram

Gully-Santiago, Michael; Herczeg, Gregory J.; Czekala, Ian; Somers, Garrett; Grankin, K. N.; Covey, Kevin R.; Donati, J.‐F.; Alencar, S. H. P.; Hussain, G. A. J.; Shappee, B. J.; Mace, Gregory N.; Lee, Jae‐Joon; Holoien, T. W. S.; Jose, Jessy; Liu, Chun-Fan

doi:10.3847/1538-4357/836/2/200

Cited by 131 publications

(125 citation statements)

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“…The sinusoidal morphology of the phase-folded light curves indicates the presence of a single large spot, similar to some other young stars with similar spectral types (e.g., Alencar et al 2010;Rebull et al 2016;Gully-Santiago et al 2017). The standard deviation of the residual of 0.11 mag is likely caused by extinction and accretion events (discussed in Sections 3.2-3.4).…”

Section: Spot Modulation In 2015-2016mentioning

confidence: 85%

“…The estimated spectral type of GI Tau ranges from K5-M0.5 (Rydgren et al 1976;Herbig 1977;Cohen & Kuhi 1979;Hartigan et al 1994;Taguchi et al 2009;Herczeg & Hillenbrand 2014), with differences caused by methodology and a non-uniform temperature distribution on the stellar surface (see, e.g., Gully-Santiago et al 2017). Extinction events have been previously detected from photometry (Herbst et al 1994;Grankin et al 2007;Rodriguez et al 2016a).…”

Section: Properties Of Gi Taumentioning

confidence: 99%

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Star–Disk Interactions in Multiband Photometric Monitoring of the Classical T Tauri Star GI Tau

Guo

Herczeg

Jose

et al. 2018

ApJ

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The variability of young stellar objects is mostly driven by star-disk interactions. In long-term photometric monitoring of the accreting T Tauri star GI Tau, we detect extinction events with typical depths of DṼ 2.5 mag that last for days to months and often appear to occur stochastically. In 2014-2015, extinctions that repeated with a quasi-period of 21 days over several months are the first empirical evidence of slow warps predicted by magnetohydrodynamic simulations to form at a few stellar radii away from the central star. The reddening is consistent with =  R 3.85 0.5 V and, along with an absence of diffuse interstellar bands, indicates that some dust processing has occurred in the disk. The 2015-2016 multiband light curve includes variations in spot coverage, extinction, and accretion, each of which results in different traces in color-magnitude diagrams. This light curve is initially dominated by a month-long extinction event and a return to the unocculted brightness. The subsequent light curve then features spot modulation with a 7.03 day period, punctuated by brief, randomly spaced extinction events. The accretion rate measured from U-band photometry ranges from´-1.3 10 8 to´-1.1 10 10 M e yr −1 (excluding the highest and lowest 5% of high-and low-accretion rate outliers), with an average of4.7 -10 9 M e yr −1 . A total of 50% of the mass is accreted during bursts of >´-12.8 10 9 M e yr -1, which indicates limitations on analyses of disk evolution using single-epoch accretion rates.

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Section: Spot Modulation In 2015-2016mentioning

confidence: 85%

Section: Properties Of Gi Taumentioning

confidence: 99%

Star–Disk Interactions in Multiband Photometric Monitoring of the Classical T Tauri Star GI Tau

Guo

Herczeg

Jose

et al. 2018

ApJ

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“…We note that the values of A V estimated from the near-infrared colors for these late-type targets by Luhman (2007) are of 0 mag for all the objects apart from LM717, which was estimated at A V = 0.4 mag (see Table A.1). Differences in the values of A V derived from near-infrared or optical spectra could be due to several physical properties of the objects, such as the presence of cold spots on the stellar surface (e.g., Stauffer et al 2003;Vacca & Sandell 2011;Pecaut 2016;Gully-Santiago et al 2017), which make the targets appear to be of later types in red spectra than in blue spectra.…”

Section: Extinction Estimatementioning

confidence: 99%

“…The absolutely flux-calibrated X-shooter spectra of our targets, with their large, simultaneous wavelength coverage and high signal-to-noise are perfectly suited to compute broad-band colors for young stars and to be used to study variations of the extinction or the presence of spots on the surface of young objects (e.g., Gully-Santiago et al 2017). This kind of study relies on the knowledge of the intrinsic colors of the star, which are gravity dependent (e.g., Luhman et al 2003).…”

Section: Photospheric Colors Of Young Starsmentioning

confidence: 99%

An extensive VLT/X-shooter library of photospheric templates of pre-main sequence stars

Manara

Frasca

Alcalá

et al. 2017

A&A

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Context. Studies of the formation and evolution of young stars and their disks rely on knowledge of the stellar parameters of the young stars. The derivation of these parameters is commonly based on comparison with photospheric template spectra. Furthermore, chromospheric emission in young active stars impacts the measurement of mass accretion rates, a key quantity for studying disk evolution. Aims. Here we derive stellar properties of low-mass (M 2 M ) pre-main sequence stars without disks, which represent ideal photospheric templates for studies of young stars. We also use these spectra to constrain the impact of chromospheric emission on the measurements of mass accretion rates. The spectra are reduced, flux-calibrated, and corrected for telluric absorption, and are made available to the community. Methods. We derive the spectral type for our targets by analyzing the photospheric molecular features present in their VLT/X-shooter spectra by means of spectral indices and comparison of the relative strength of photospheric absorption features. We also measure effective temperature, gravity, projected rotational velocity, and radial velocity from our spectra by fitting them with synthetic spectra with the ROTFIT tool. The targets have negligible extinction (A V < 0.5 mag) and spectral type from G5 to K6, and from M6.5 to M8. They thus complement the library of photospheric templates presented in our previous publication. We perform synthetic photometry on the spectra to derive the typical colors of young stars in different filters. We measure the luminosity of the emission lines present in the spectra and estimate the noise due to chromospheric emission in the measurements of accretion luminosity in accreting stars. Results. We provide a calibration of the photospheric colors of young pre-main sequence stars as a function of their spectral type in a set of standard broad-band optical and near-infrared filters. The logarithm of the noise on the accretion luminosity normalized to the stellar luminosity is roughly constant and equal to ∼−2.3 for targets with masses larger than 1 solar mass, and decreases with decreasing temperatures for lower-mass stars. For stars with masses of ∼1.5 M and ages of ∼1−5 Myr, the chromospheric noise converts to a limit of measurable mass accretion rates of ∼3 × 10 −10 M /yr. The limit on the mass accretion rate set by the chromospheric noise is of the order of the lowest measured values of mass accretion rates in Class II objects.

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“…For example, using the Matérn kernel for the t-correlation might provide a Gaussian process that more closely reflects the stochastic accretion process. Such a modeling approach could also be combined with a mean function f m l ( ) specified by flexible physics-based models (e.g., Czekala et al 2015;Gully-Santiago et al 2017) in order to simultaneously determine the fundamental properties of the veiled star, a traditionally difficult process (Herczeg & Hillenbrand 2014).…”

Section: Extensions To Time-variable Spectramentioning

confidence: 99%

Disentangling Time-series Spectra with Gaussian Processes: Applications to Radial Velocity Analysis

Czekala

Mandel

Andrews

et al. 2017

ApJ

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Measurements of radial velocity variations from the spectroscopic monitoring of stars and their companions are essential for a broad swath of astrophysics; these measurements provide access to the fundamental physical properties that dictate all phases of stellar evolution and facilitate the quantitative study of planetary systems. The conversion of those measurements into both constraints on the orbital architecture and individual component spectra can be a serious challenge, however, especially for extreme flux ratio systems and observations with relatively low sensitivity. Gaussian processes define sampling distributions of flexible, continuous functions that are well-motivated for modeling stellar spectra, enabling proficient searches for companion lines in time-series spectra. We introduce a new technique for spectral disentangling, where the posterior distributions of the orbital parameters and intrinsic, rest-frame stellar spectra are explored simultaneously without needing to invoke crosscorrelation templates. To demonstrate its potential, this technique is deployed on red-optical time-series spectra of the mid-M-dwarf binary LP661-13. We report orbital parameters with improved precision compared to traditional radial velocity analysis and successfully reconstruct the primary and secondary spectra. We discuss potential applications for other stellar and exoplanet radial velocity techniques and extensions to time-variable spectra. The code used in this analysis is freely available as an open-source Python package.

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Placing the Spotted T Tauri Star LkCa 4 on an HR Diagram

Cited by 131 publications

References 113 publications

Star–Disk Interactions in Multiband Photometric Monitoring of the Classical T Tauri Star GI Tau

Star–Disk Interactions in Multiband Photometric Monitoring of the Classical T Tauri Star GI Tau

An extensive VLT/X-shooter library of photospheric templates of pre-main sequence stars

Disentangling Time-series Spectra with Gaussian Processes: Applications to Radial Velocity Analysis

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