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Added by
Nicole Arulanantham
Publication date
2016
fields
Physics
and research's language is
English
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We examine the light and color evolution of the T Tauri binary KH 15D through photometry obtained at wavelengths between 0.55 and 8.0 $mu$m. The data were collected with ANDICAM on the 1.3 m SMARTS telescope at Cerro-Tololo Inter-American Observatory and with IRAC on the Spitzer Space Telescope. We show that the systems circumbinary ring, which acts as a screen that covers and uncovers different portions of the binary orbit as the ring precesses, has reached an orientation where the brighter component (star B) fully or nearly fully emerges during each orbital cycle. The fainter component (star A) remains fully occulted by the screen at all phases. The leading and trailing edges of the screen move across the sky at the same rate of $sim$15 meters per second, consistent with expectation for a ring with a radius and width of $sim$4 AU and a precession period of $sim$6500 years. Light and color variations continue to indicate that the screen is sharp edged and opaque at emph{VRIJH} wavelengths. However, we find an increasing transparency of the ring edge at 2.2, 3.6, and 4.5 $mu$m. Reddening seen at the beginning of the eclipse that occurred during the CSI 2264 campaign particularly suggests selective extinction by a population of large dust grains. Meanwhile, the gradual bluing observed while star B is setting is indicative of forward scattering effects at the edge of the ring. The SED of the system at its bright phase shows no evidence of infrared excess emission that can be attributed to radiation from the ring or other dust component out to 8 microns.
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Following two years of complete occultation of both stars by its opaque circumbinary ring, the binary T Tauri star KH 15D has abruptly brightened again during apastron phases, reaching I = 15 mag. Here, we show that the brightening is accompanied by a change in spectral class from K6/K7 (the spectral class of star A) to ~K1, and a bluing of the system in V-I by about 0.3 mag. A radial velocity measurement confirms that, at apastron, we are now seeing direct light from star B, which is more luminous and of earlier spectral class than star A. Evidently, the trailing edge of the occulting screen has just become tangent to one anse of star Bs projected orbit. This confirms a prediction of the precession models, supports the view that the tilted ring is self-gravitating, and ushers in a new era of the systems evolution that should be accompanied by the same kind of dramatic phenomena observed from 1995-2009. It also promotes KH 15D from a single-lined to a double-lined eclipsing binary, greatly enhancing its value for testing pre-main sequence models. The results of our study strengthen the case for truncation of the outer ring at around 4 AU by a sub-stellar object such as an extremely young giant planet. The system is currently at an optimal configuration for detecting the putative planet and we urge expedient follow-up observations.
KH 15D is a well-known spectroscopic binary because of its unique and dramatic photometric variability. The variability is explained by a circumbinary dust ring but the ring itself was never directly detected. We present a new interpretation of the double-peaked [O~{scriptsize I}],$lambda$6300 profiles as originating from the hot disk surface of KH 15D. By modeling these profiles, we measure emitting radii between $sim$0.5--5 au, basically a gaseous ring very similar in radial extent to the dust ring inferred from modeling the systems photometric variability. We also discuss the possibility that external photoevaporation driven by UV photons from the nearby massive star HD~47887 has truncated the outer edge of the disk to the observed value.
In recent years, an increasing number of publications have been addressed to the peculiar and mysterious pre-main sequence star V582 Mon, also known as KH 15D. This extraordinary T Tauri star, located in the young star cluster NGC 2264, appears as to be an eclipsing variable. In the present paper, we report a unique and self-consistent set of light curves in the blue and near-infrared bands, spanning a 15-year interval (epoch 1955-1970). Our photometric data show clearly the beginning of the eclipse stage occurred in early 1958 in the blue, and perhaps around four years later in the infrared. The light curve period turns out to be the same reported by recent observations (about 48.3 days), so that no evidence for a period change results. On the other hand, in our data the light curve shape appears as sinusoidal and is therefore different from the one displayed today. The photometric behaviour, determined with time-series and colour-index analysis, suggests that V582 Mon (KH 15D) could be initially surrounded by an accretion disk/torus seen edge-on, with subsequent thin dust formation at the beginning of the blue radiation absorption. The dust could then aggregate into larger particles providing the transition between selective and total absorption, accompanied with eclipsing variability in the infrared. The minima of the periodic light curve become deeper due to the increasing dimension and number of dust grains, and then flattens due to a contraction in the disk.
An extensive photometric monitoring of KH 15D, an enigmatic variable in the young star cluster NGC 2264, has been conducted. Simultaneous and accurate near-infrared (JHKs-bands) photometry is presented between 2003 December and 2005 March covering most of the variable phase. The infrared variability is characterized by large-amplitude and long-lasting eclipse, as observed at optical. The period of variability is 48.3 +/- 0.2 days, the maximum photometric amplitude of variability is ~4.2 mag, and the eclipse duration is ~0.5 in phase units. These are consistent with the most recent period, amplitude, and duration at optical. The blueing of the J-H color (~0.16 mag) during the eclipse, which has been suggested before, is unambiguously confirmed; a similar blueing at H-Ks is less clear but is probably present at a similar level. The overall shape of the JHKs light curves is very similar to the optical one, including a fair time-symmetry and a less stable flux during the eclipse with a slight hump near the zero phase. Most of these variability features of KH 15D observed at near-infrared wavelengths can be explained with the recent model employing an eclipse by the inclined, precessing disk and an outer scattering region around a pre-main-sequence binary.
Na I D lines in the spectrum of the young binary KH 15D have been analyzed in detail. We find an excess absorption component that may be attributed to foreground interstellar absorption, and to gas possibly associated with the solids in the circumbinary disk. The derived column density is log N_NaI = 12.5 cm^-2, centered on a radial velocity that is consistent with the systemic velocity. Subtracting the likely contribution of the ISM leaves log N_NaI ~ 12.3 cm^-2. There is no detectable change in the gas column density across the knife edge formed by the opaque grain disk, indicating that the gas and solids have very different scale heights, with the solids being highly settled. Our data support a picture of this circumbinary disk as being composed of a very thin particulate grain layer composed of millimeter-sized or larger objects that are settled within whatever remaining gas may be present. This phase of disk evolution has been hypothesized to exist as a prelude to the formation of planetesimals through gravitational fragmentation, and is expected to be short-lived if much gas were still present in such a disk. Our analysis also reveals the presence of excess Na I emission relative to the comparison spectrum at the radial velocity of the currently visible star that plausibly arises within the magnetosphere of this still-accreting young star.
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