Exploring the color signatures of different physical scenarios

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The T Tauri phenomenon across the spectrum

T Tauri stars were first detected and categorized based on observational data in the optical ; however, it soon became evident that distinctive features of these objects appear over a much broader wavelength domain.
At the time of Herbig’s (1962) review, it was already known that these sources typically display a flux excess in the blue and, particularly, the ultraviolet regions of their spectra with respect to their photospheric colors (e.g., Haro & Herbig 1955; Walker 1956). Observations also showed a tendency for ultraviolet excess to accompany intense line emission. Interpretation of this feature would have to await Walker’s (1972) insightful analysis of ultraviolet-excess stars in the Orion Nebula Cluster and in NGC 2264. He suggested that the blue continuum and emission-line spectrum exhibited by these objects are produced in zones, at the stellar surface, impacted by infalling material from the circumstellar environment. Interestingly, the author considered two possible scenarios for the material infall onto the star: one where it is spherically symmetric, and a second where it is confined to a region of moderate vertical extent around the star. This represented the first study to prepare the ground for the association of the UV excess in T Tauri stars with accretion of material from a circumstellar disk.
Mendoza V. (1966) reported multicolor photometric investigation of 26 TTS, on wavelengths ranging from the ultraviolet (U -band, 0.36 µm) to the near-infrared (M -band, 5 µm). His study confirmed the presence of a characteristic ultraviolet excess for the majority of his targets. To the surprise of the author, data also showed a significant infrared excess for all the stars in the sample, with values up to several magnitudes above the intrinsic, spectral type-dependent, colors. The hypothesis contemplated to explain this feature invoked a composite system with a small stellar core and a large surrounding envelope; emission from the first component would dominate the short-wavelength photometry, while long-wavelength photometry would take sig-nificant contribution from thermal re-radiation from the envelope (Mendoza V. 1968). This observational picture matched the global predictions of first hydrodynamical models of collaps-ing protostellar clouds and PMS evolution (e.g. Larson 1969): namely that, at a given moment in its early evolution, the “young stellar object” (YSO; Strom 1972) would consist of a central core surrounded by a circumstellar envelope of gas and dust, gradually accreted by the core. A few years later, Cohen (1973) explored the photometric properties of T Tauri stars in the mid-infrared (2–20 µm) and drew similar conclusions: thermal emission from circumstellar dust was designated as the possible source of the long-wavelength brightness detected for most of the objects in the study (about 30 from various star-forming regions). In the following years, the interpretation of dust, rather than gas, as being responsible for the near- and mid-infrared ex-cess emission observed for T Tauri stars met with growing evidence (e.g. Rydgren & Vrba 1981), and was ultimately supported by similar investigation of YSO spectra in the far-infrared (up to 100 µm; Rucinski 1985). Gaseous emission models could not reproduce the behavior (i.e., the λ-dependence) of the infrared emitted spectrum (spectral energy distribution, or SED) of these objects, as reconstructed from observations; hence, solid grains were retained as the most likely responsible for TTS infrared emission from ∼2 µm to 2 mm (Hartmann 1998). This conclusion also provided compelling evidence for a non-isotropic distribution of dust particles around the star. If the amount of dust required to produce the observed infrared excess were distributed in a spherical shell around the central protostar, this would determine a huge visual extinction AV (Beckwith et al. 1990), a factor of 102 − 103 higher than the values actually observed, which would practically conceal the central source in the optical. The clear inconsistency between these theoretical inferences and the observations indicated that dust must instead be distributed highly non-uniformly, in a flattened region around the star, so that there is no significant concen-tration of dusty material along the line of sight to the object in most geometrical configurations. This circumstellar material is contained within a few hundred AU of the central star (Terebey et al. 1993), as indicated by interferometric maps of millimetric continuum emission (e.g. Keene & Masson 1990, see also Fig. 1.1b); these features are all consistent with a picture of disk-like distribution of material around the star.

T Tauri phase in the paradigm of low-mass early stellar evolution

The exploration of the X-ray domain started in the late ’70s with the launch of the Einstein Ob-servatory. Ku & Chanan (1979) were the first to suggest that T Tauri stars might be responsible for the X-ray emission detected in the Orion Nebula. These first indications on TTS being X-ray emitters were developed and confirmed in the studies of Walter & Kuhi (1981, 1984). Authors reported investigation of 23 known pre-main sequence objects in different fields, and detection of X-ray emission in about one third of cases. They also noticed that most detections occurred for stars with relatively low Hα equivalent width (EW) in their sample. X-ray emission was interpreted to arise in a small region close to the photosphere (corona).
The characterization of activity in the X-rays marked a most important step in the study of T Tauri stars. These works revealed the ubiquitous presence of a large population of PMS stars, clearly identified as such based on their spatial association with dark clouds, the strong Lithium absorption and the location above the MS track on the HR diagram, which displayed a strong X-ray emission but did not fit in the picture of a “classical” T Tauri stars (CTTS), i.e., they did not show IR excess nor strong line emission (Walter 1986). These observations, indicative of the absence of circumstellar material around these objects, led to the definition of a new, complementary class of PMS objects, that Walter defined “naked” T Tauri stars. A spectroscopic definition for these stars (Hα EW < 10 Å) was proposed in the catalog of Herbig & Bell (1988), who introduced the notation of “weak-line T Tauri stars” (WTTS). WTTS and CTTS were suggested to represent “two components of a single population of low-mass PMS stars”, with no sharp transition but a “continuum of spectral properties” connecting the most extreme cases in the two groups (Walter et al. 1988).
The discovery of WTTS was instrumental in achieving a deeper understanding of the T Tauri nature. WTTS were interpreted as CTTS’ subsequent evolutionary step toward the Main Se-quence3. The T Tauri phenomenon soon appeared as “one aspect of stellar formation that can no longer be treated independently of other aspects” (Bertout 1989); the “T Tauri” phase in star formation was defined as stage where the infalling envelope is cleared and the previously embedded protostar+disk system becomes optically visible (Shu & Adams 1987). This is the last step of the star formation process, which gives birth to a young solar-type star.

A more realistic picture of magnetospheric accretion

The depiction of the magnetospheric accretion process in Fig. 1.6, although conveying the basic physical principles, is an oversimplification of the actual dynamics of disk accretion in CTTS. The assumption of axisymmetry, for instance, is not met in the reality: magnetic map reconstruc-tion for several CTTS from Zeeman-Doppler imaging (see Gregory et al. 2012) has shown that misalignments of a few ×10◦ between the stellar magnetic axis and rotation axis are common. Fig. 1.7 illustrates the magnetospheric accretion pattern expected, from 3D simulations (Ro-manova et al. 2004), in the case of a dipolar magnetic field slightly inclined with respect to the rotation axis of the system. In this configuration, the disk is still disrupted at the truncation radius rT , but the morphology of the magnetospheric accretion flow is more complex. For small misalignment angles Θ between the rotation axis and the magnetic axis (Θ . 30◦), matter typ-ically accretes in two streams. These are located at opposite latitudes, as the accretion columns tend to form at the regions where the magnetic poles are closest to the disk plane. These two “curtains” of accretion determine the formation of a bow-shaped accretion shock close to the magnetic poles. In the inclined dipole configuration, the inner disk regions tend to be warped at the base of the two accretion columns (e.g., Bouvier et al. 1999), as a result of the dynamical disk-magnetosphere interaction, which determines a thickening of the inner regions of the disk with accumulation of material close to the magnetosphere; this geometrically translates to the formation of two trailing spiral arms in the case of accretion occurring in two separate streams for an inclined dipole (Bouvier et al. 2007b).

The manifold variability of T Tauri stars

Photometric investigations of the physics of T Tauri systems take advantage of two complemen-tary types of diagnostics: characterizing the colors of the stars (i.e., probing the wavelength domain) and exploring stellar variability (i.e., probing the time domain). Colors are a tracer of the slope of the SED in different regions of the spectrum, and hence provide insight into the dominant components of the emission and, therefore, into the physical mechanisms at the base of the observed photometric properties. Monitoring photometric variability enables studies of the dynamics of evolution and of the relevant timescales for the physical processes of interest to the dominant features of the systems.
As discussed in Sect. 1.1, prominent variability is a defining mark of T Tauri stars. Photo-metric variations are observed on a wide range of wavelengths (X-rays, optical, infrared) and on all timescales investigated so far (from . hours to days, months and years). Besides, young stars display strong spectroscopic variability, and variations of the polarization in TTS, indicative of the distribution of circumstellar dust, are documented as well (e.g., review by Appenzeller & Mundt 1989).
In the following, I will focus on the different flavors of photometric variability exhibited by young stars in the optical and at short wavelengths. This probes the properties of the stellar photosphere and the star-disk close environment. Sect. 1.4.1 explores the types of light variations observed on timescales from days to weeks; in Sect. 1.4.2, I will present the nature of variability on much shorter (hours) and longer (years) timescales.

Variability on mid-term (days to weeks) timescales

In 1994, Herbst et al. presented a comprehensive catalog of U BV RI light curves for 80 young stars. This gathered photometric data obtained on various telescopes and surveys over a time span of several years, with tens of epochs (light curve points) for each source. The comparative analysis of this sample enabled the authors to identify four main types of photometric behaviors (see Fig, 1.9), corresponding to distinct physical processes that would dominate the observed variability pattern for different groups.
Type I variability, responsible for the well-behaved, periodic light curves observed for WTTS, consists in a simple rotational modulation effect produced by a non-homogeneous distribution of dark cool spots at the stellar surface, whose nature is linked to the underlying magnetic activity of the stars. Starspots are assumed to develop at the foot points of magnetic loops, where convective motions are inhibited by the strong magnetic fields; hence, the surface distribution of cold spots may provide indications on the geometry and complexity of the stellar magnetic field. Photometric amplitudes detected for Type I-variables are moderate, ranging from . 0.1 to ∼ 0.5 mag in the optical, and decline from bluer to redder wavelengths; this occurs because the contrast between the cool spot distribution and the photosphere diminishes toward longer wavelengths (see Fig. 4.11 in Chapter 4 of this thesis). The overall light curve pattern remains stable over tens to hundreds of rotational cycles (i.e., over months to years), which suggests that such spots are long-lived, although the exact shape of the light curve “unit” (one period) may vary from one cycle to the other, reflecting corresponding changes in the spot distribution and properties.
The optical variability of WTTS can be entirely accounted for by cold spot modulation (additional sources of variability associated with chromospheric activity, i.e. flares, may also be present at shorter wavelengths on the shorter term). Cold spots are also expected at the surface of accreting CTTS, which are similarly magnetically active; however, cold spot modulation represents only a single, underlying component of variability in these cases, on top of which more prominent flux variations of different origin shape the light curve pattern. The interplay between various sources of variability for CTTS is at the base of the huge diversity in light curve morphology detected across the group, with large (up to a few mag) photometric amplitudes and often irregular profiles, where semi-periodic components coexist with rapid, stochastic flux variations and fading events.


MegaCam observing runs on NGC 2264

Starting on February 14, 2012, and for a period of two weeks, we monitored the u-band and r-band variability of objects in the NGC 2264 field with MegaCam, in the context of the CSI 2264 campaign. The target was observed during 11 nights distributed along the 14-day-long run; among these, 7 nights were photometric. A single u + r observing block, consisting of five dithered exposures in u and five dithered exposures in r, obtained consecutively, was performed repeatedly on each observing night during the run, at a cadence varying from 20 min to 1.5 h. Individual exposures performed were of 3 s in the r-band and of 60 s in the u-band, for a total integration time of 15 s in r and 300 s in u in each observing sequence. The 5-step dithering pattern adopted allowed us to compensate for the presence of bad pixels and small gaps on the CCD mosaic, although large gaps are still present on the final assembled images of the region. On February 28, 2012, we also obtained a deep mapping of the region in the i-band. As for u- and r-band observations, a 5-step dithering pattern was used; the observing sequence was performed in two different exposure modes, a first with integration time of 180 s for single exposure, aimed at detecting all sources in the FOV, and a second with short exposures of 5 s, to recover the bright sources saturated in the long exposure mode.

The colors of young stars at short wavelengths

Color indices, or colors, are very informative regarding the nature of the observed young stellar systems. Optical colors provide indications on the photospheric temperature, hence spectral type, of the object; bright UV colors may indicate the presence of hot, overluminous regions on top of the stellar surface; IR colors enable investigation of the dusty circumstellar environment. Contrary to apparent magnitudes, colors do not depend on the distance to the objects; these can then be more immediately compared to characterize the physical properties of different stellar groups, with the caveat that a displacement toward redder colors on a color-color diagram may be caused by interstellar and/or circumstellar extinction.

The color loci of field stars in the SDSS system

In order to investigate the photometric properties of young stars, it is of interest to define the photospheric color properties expected for main-sequence (MS) stars.
Covey et al. (2007) analyzed a sample of over 300 000 little extincted (Ar < 0.2) point sources, detected during the earliest stages of the SDSS survey, to map the dwarf color loci in ugri filters (Fig. 3.1). They calculated synthetic colors using spectral standards from the compilation of Pickles (1998) and derived an empirical scale of SDSS colors as a function of spectral type. An independent scale was proposed by Kraus & Hillenbrand (2007); this is derived by assembling a set of empirical SED models for members of the open cluster Praesepe and then fitting those models to derive a scale of colors and absolute magnitudes as a function of spectral type.
In Fig. 3.2, we compare the color distribution of point sources detected in the MegaCam field with the reference color sequences for solar-metallicity dwarfs compiled by Covey et al. (2007) and Kraus & Hillenbrand (2007). The color loci defined by the bulk of field stars are found to match closely those defined in Covey et al. (2007), illustrated in Fig. 3.1.
In Fig. 3.2a, earlier-type field stars are located along the diagonal ellipse: foreground or slightly reddened objects define the lower (bluer) part of the distribution, while more extincted objects and background stars trace the continuation1 of the distribution, along the direction of the reddening vector, above the horizontal branch at g−r ∼ 1.4. M-type stars are predominantly located on the horizontal branch at a constant g − r of about 1.4; this property is due to the strong TiO bands present in their spectra (Finlator et al. 2000). Their r − i color, on the other hand, varies strongly with spectral subclass.
Similarly, a progression of increasing spectral types and reddening effects can be observed from left to right along the ellipse in Fig. 3.2b; M-type stars are expected to concentrate along the vertical slice centered on g−r ∼ 1.4 (as indicated by the empirical color – spectral type sequences overplotted to the data point distribution), while reddened early to K-type stars populate the right end of the elliptical distribution.
An offset of a few tenths of a mag is observed between the mean locus of field stars in our survey and the sequences of Covey et al. (2007) and Kraus & Hillenbrand (2007). This offset cannot be explained with imprecisions in our calibration. A possible reason for this discrepancy may lie in a metallicity lower than solar standards for the bulk of field stars probed here. Several studies have determined a negative metallicity gradient from the center to the outer regions of the Galaxy disk (Δ[Fe/H]/ΔR ≃ −0.07 dex/kpc; e.g., Bergemann et al. 2014). Metal-poorer stars are less affected from the line-blanketing effect, and this translates to bluer u − g colors (Covey et al. 2007; Ivezić et al. 2008).

Table of contents :

1 Introduction 
1.1 On the origin of the T Tauri case
1.1.1 A new class of young, variable stars
1.1.2 The T Tauri phenomenon across the spectrum
1.1.3 T Tauri phase in the paradigm of low-mass early stellar evolution
1.2 Disks around young stars
1.2.1 Star formation in a nutshell
1.2.2 Structure and physics of circumstellar disks
1.3 Disk accretion in T Tauri stars
1.3.1 Magnetospheric accretion
1.3.2 Disk–locking?
1.4 The manifold variability of T Tauri stars
1.4.1 Variability on mid-term (days to weeks) timescales
1.4.2 Variability on shorter and longer timescales
1.4.3 The space-borne revolution in YSO variability studies
1.5 Open issues in disk accretion from an observational perspective
1.5.1 Aim and outline of this thesis
2 The Coordinated Synoptic Investigation of NGC 2264 
2.1 The young open cluster NGC 2264
2.2 The CSI 2264 project
2.2.1 Overview of the observing campaign
2.2.2 CFHT dataset
2.3 CSI 2264: a synthesis
2.4 Specific contribution from this thesis
3 Mapping the different accretion regimes in NGC 2264 
3.1 The colors of young stars at short wavelengths
3.1.1 The color loci of field stars in the SDSS system
3.1.2 Colors and UV excess of young stars
3.1.3 UV excess vs. different accretion diagnostics
3.2 A UV census of the NGC 2264 young stellar population
3.2.1 New CTTS candidates in NGC 2264
3.2.2 Field contaminants in the NGC 2264 sample
3.3 Derivation of individual stellar parameters
3.3.1 Individual AV estimates
3.3.2 Spectral types and effective temperatures
3.3.3 Bolometric luminosities
3.3.4 Stellar masses and radii
3.4 UV excess and mass accretion rates
3.4.1 Measuring the UV flux excess
3.4.2 From u-band excess luminosity to total accretion luminosity
3.4.3 Mass accretion rates
3.5 Accretion regimes in NGC 2264
3.5.1 The ˙Macc −M⋆ relationship
3.5.2 Accretion variability
3.5.3 Different accretion regimes/mechanisms
3.5.4 Evolutionary spread across the cluster
3.6 Conclusions
4 The UV variability of young stars in NGC 2264 
4.1 A closer look at photometric variability in the CFHT sample
4.1.1 Measuring the variability of CTTS and WTTS: light curve rms
4.1.2 Measuring the variability of CTTS and WTTS: Stetson’s index J
4.2 The imprints of disk accretion in UV variability
4.2.1 A comparison between UV excess and u-band variability
4.2.2 Time evolution on the r vs. u − r diagram of the cluster
4.2.3 Exploring the color signatures of different physical scenarios
4.2.4 A global picture of color variability for different YSO types
4.3 A spot model description of YSO variability
4.3.1 Formulation of the spot model
4.3.2 Implementation of the model
4.3.3 A global picture of spot properties for TTS in NGC 2264
4.3.4 The different nature of modulated variability for CTTS vs. WTTS
4.4 Timescales of variability for the accretion process
4.5 Conclusions
5 The accretion–rotation connection in young stars 
5.1 Photometric period determination
5.1.1 Period-search methods used in this study
5.1.2 Implementation of the period-search routine
5.2 Results
5.2.1 Period distribution for NGC 2264: CTTS vs. WTTS
5.2.2 Mass dependence?
5.2.3 Are CTTS periods similar in nature to WTTS periods?
5.3 The accretion–rotation connection
5.4 Conclusions
6 Conclusions and perspectives 
6.1 Case of study: a brief recap
6.2 Main points of this work
6.2.1 Different accretion regimes coexist within the cluster
6.2.2 ˙Macc reflect a diversity in accretion mechanisms and cluster evolution
6.2.3 Variability in young stars has a broadly assorted nature
6.2.4 Timescales of days dominate the variability of WTTS and CTTS
6.2.5 Disks have an impact on the rotation properties of young stars
6.3 Perspectives


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