Spectropolarimetry and Modeling of the Eclipsing T Tauri Star KH 15D

KH 15D is a strongly variable T Tauri star in the young star cluster NGC 2264 that shows a decrease in flux of 3.5 magnitudes lasting for 18 days and repeating every 48 days. The eclipsing material is likely due to orbiting dust or rocky bodies in a partial ring or warped disk that periodically occults the star. We measured the polarized spectrum in and out of eclipse at the Keck and Palomar observatories. Outside of the eclipse, the star exhibited low polarization consistent with zero. During eclipse, the polarization increased dramatically to ~2% across the optical spectrum, while the spectrum had the same continuum shape as outside of eclipse and exhibited emission lines of much larger equivalent width, as previously seen. From the data, we conclude that (a) the scattering region is uneclipsed; (b) the scattering is nearly achromatic; (c) the star is likely completely eclipsed so that the flux during eclipse is entirely due to scattered light, a conclusion also argued for by the shape of the ingress and egress. We argue that the scattering is not due to electrons, but may be due to large dust grains of size ~10 micron, similar to the interplanetary grains which scatter the zodiacal light. We construct a warped-disk model with an extended dusty atmosphere which reproduces the main features of the lightcurve, namely (a) a gradual decrease before ingress due to extinction in the atmosphere (similar for egress); (b) a sharper decrease within ingress due to the optically-thick base of the atmosphere; (c) a polarized flux during eclipse which is 0.1% of the total flux outside of eclipse, which requires no fine-tuning of the model. (abridged)Comment: 9 pages, 7 figures, accepted for publication in ApJ, MPEG simulation available at http://www.astro.washington.edu/agol/scatter2.mp

The Distribution of Stellar Mass in the Pleiades

As part of an effort to understand the origin of open clusters, we present a statistical analysis of the currently observed Pleiades. Starting with a photometric catalog of the cluster, we employ a maximum likelihood technique to determine the mass distribution of its members, including single stars and both components of binary systems. We find that the overall binary fraction for unresolved pairs is 68%. Extrapolating to include resolved systems, this fraction climbs to about 76%, significantly higher than the accepted field-star result. Both figures are sensitive to the cluster age, for which we have used the currently favored value of 125 Myr. The primary and secondary masses within binaries are correlated, in the sense that their ratios are closer to unity than under the hypothesis of random pairing. We map out the spatial variation of the cluster's projected and three-dimensional mass and number densities. Finally, we revisit the issue of mass segregation in the Pleiades. We find unambiguous evidence of segregation, and introduce a new method for quantifying it.Comment: 41 pages, 14 figures To Be Published in The Astrophysical Journa

Polarimetric variations of binary stars. V. Pre-main-sequence spectroscopic binaries located in Ophiuchus and Scorpius

We present polarimetric observations of 7 pre-main-sequence (PMS) spectroscopic binaries located in the rho Oph and Upper Sco star forming regions (SFRs). The average observed polarizations at 7660A are between 0.5% and 3.5%. After estimates of the interstellar polarization are removed, all binaries have an intrinsic polarization above 0.4%. Two binaries, NTTS162814-2427 and NTTS162819-2423S, present high levels of intrinsic polarization between 1.5% and 2.1%. All 7 PMS binaries have a statistically variable or possibly variable polarization. Combining these results with our previous sample of binaries located in the Tau, Aur and Ori SFRs, 68% of the binaries have an intrinsic polarization above 0.5%, and 90% of the binaries are polarimetrically variable or possibly variable. NTTS160814-1857, NTTS162814-2427, and NTTS162819-2423S are clearly polarimetrically variable. The first two also exhibit phase-locked variations over ~10 and ~40 orbits respectively. NTTS160905-1859 shows periodic variations that are not phased-locked and only present for short intervals of time. The amplitudes of the variations reach a few tenths of a percent. The high-eccentricity system NTTS162814-2427 shows single-periodic variations, in agreement with our previous numerical simulations. Non-periodic events introduce stochastic noise that partially masks the periodic variations and prevents the Brown, McLean, & Emslie (1978) formalism from finding a reasonable estimate of the inclination.Comment: 63 pages, including 21 figures and 18 tables, accepted by A

Polarimetric variations of binary stars. IV. Pre-main-sequence spectroscopic binaries located in Taurus, Auriga, and Orion

We present polarimetric observations of 14 pre-main-sequence (PMS) binaries located in the Taurus, Auriga, and Orion star forming regions. The majority of the average observed polarizations are below 0.5%, and none are above 0.9%. After removal of estimates of the interstellar polarization, about half the binaries have an intrinsic polarization above 0.5%, even though most of them do not present other evidences for the presence of circumstellar dust. Various tests reveal that 77% of the PMS binaries have or possibly have a variable polarization. LkCa3, Par1540, and Par2494 present detectable periodic and phase-locked variations. The periodic polarimetric variations are noisier and of a lesser amplitude (~0.1%) than for other types of binaries, such as hot stars. This could be due to stochastic events that produce deviations in the average polarization, a non-favorable geometry (circumbinary envelope), or the nature of the scatterers (dust grains are less efficient polarizers than electrons). Par1540 is a Weak-line TTauri Star, but nonetheless has enough dust in its environment to produce detectable levels of polarization and variations. A fourth interesting case is W134, which displays rapid changes in polarization that could be due to eclipses. We compare the observations with some of our numerical simulations, and also show that an analysis of the periodic polarimetric variations with the Brown, McLean, & Emslie (BME) formalism to find the orbital inclination is for the moment premature: non-periodic events introduce stochastic noise that partially masks the periodic low-amplitude variations and prevents the BME formalism from finding a reasonable estimate of the orbital inclination.Comment: 70 pages, 20 figures, to be published in the Astronomical Journa

Abundances of the elements in the solar system

A review of the abundances and condensation temperatures of the elements and their nuclides in the solar nebula and in chondritic meteorites. Abundances of the elements in some neighboring stars are also discussed.Comment: 42 pages, 11 tables, 8 figures, chapter, In Landolt- B\"ornstein, New Series, Vol. VI/4B, Chap. 4.4, J.E. Tr\"umper (ed.), Berlin, Heidelberg, New York: Springer-Verlag, p. 560-63

Modern optical astronomy: technology and impact of interferometry

The present `state of the art' and the path to future progress in high spatial resolution imaging interferometry is reviewed. The review begins with a treatment of the fundamentals of stellar optical interferometry, the origin, properties, optical effects of turbulence in the Earth's atmosphere, the passive methods that are applied on a single telescope to overcome atmospheric image degradation such as speckle interferometry, and various other techniques. These topics include differential speckle interferometry, speckle spectroscopy and polarimetry, phase diversity, wavefront shearing interferometry, phase-closure methods, dark speckle imaging, as well as the limitations imposed by the detectors on the performance of speckle imaging. A brief account is given of the technological innovation of adaptive-optics (AO) to compensate such atmospheric effects on the image in real time. A major advancement involves the transition from single-aperture to the dilute-aperture interferometry using multiple telescopes. Therefore, the review deals with recent developments involving ground-based, and space-based optical arrays. Emphasis is placed on the problems specific to delay-lines, beam recombination, polarization, dispersion, fringe-tracking, bootstrapping, coherencing and cophasing, and recovery of the visibility functions. The role of AO in enhancing visibilities is also discussed. The applications of interferometry, such as imaging, astrometry, and nulling are described. The mathematical intricacies of the various `post-detection' image-processing techniques are examined critically. The review concludes with a discussion of the astrophysical importance and the perspectives of interferometry.Comment: 65 pages LaTeX file including 23 figures. Reviews of Modern Physics, 2002, to appear in April issu

Study of the reaction e^{+}e^{-} -->J/psi\pi^{+}\pi^{-} via initial-state radiation at BaBar

We study the process $e^+e^-\to J/\psi\pi^{+}\pi^{-}$ with initial-state-radiation events produced at the PEP-II asymmetric-energy collider. The data were recorded with the BaBar detector at center-of-mass energies 10.58 and 10.54 GeV, and correspond to an integrated luminosity of 454 $\mathrm{fb^{-1}}$. We investigate the $J/\psi \pi^{+}\pi^{-}$ mass distribution in the region from 3.5 to 5.5 $\mathrm{GeV/c^{2}}$. Below 3.7 $\mathrm{GeV/c^{2}}$ the $\psi(2S)$ signal dominates, and above 4 $\mathrm{GeV/c^{2}}$ there is a significant peak due to the Y(4260). A fit to the data in the range 3.74 -- 5.50 $\mathrm{GeV/c^{2}}$ yields a mass value $4244 \pm 5$ (stat) $\pm 4$ (syst)$\mathrm{MeV/c^{2}}$ and a width value $114 ^{+16}_{-15}$ (stat)$\pm 7$(syst)$\mathrm{MeV}$ for this state. We do not confirm the report from the Belle collaboration of a broad structure at 4.01 $\mathrm{GeV/c^{2}}$. In addition, we investigate the $\pi^{+}\pi^{-}$ system which results from Y(4260) decay

IC 4665 DANCe I. Members, empirical isochrones, magnitude distributions, present-day system mass function, and spatial distribution

Context. The study of star formation is extremely challenging, due to the lack of complete and clean samples of young nearby clusters and star-forming regions. The recent Gaia DR2 catalogue complemented with the deep ground-based COSMIC DANCe catalogue o ers a new database of unprecedented accuracy to revisit the membership of clusters and star-forming regions. The 30 Myr open cluster IC 4665 is one of the few well-known clusters of this age and it is an excellent target where evolutionary models can be tested and planetary formation studied. Aims. We provide a comprehensive membership analysis of IC 4665 and study the following properties: empirical isochrones, distance, magnitude distribution, present-day system mass function, and spatial distribution. Methods. We used the Gaia DR2 catalogue together with the DANCe catalogue to look for members via a probabilistic model of the distribution of the observable quantities in both the cluster and background populations. Results. We obtained a final list of 819 candidate members that cover a 12.4 magnitude range (7 < J < 19:4). We find that 50% are new candidates, and we estimate a conservative contamination rate of 20%. This unique sample of members allows us to obtain a present-day system mass function in the range of 0.02–6 M , which reveals a number of details not seen in previous studies. In addition, we find that a spherically symmetric spatial distribution is favoured by our final list of members for this young open cluster. Conclusions. Our membership analysis represents a significant increase in the quantity and quality (low contamination) with respect to previous studies. It o ers an excellent opportunity to revisit other fundamental parameters such as age

A UV-to-NIR Study of Molecular Gas in the Dust Cavity around RY Lupi

We present a study of molecular gas in the inner disk (r ∼ 0.4± 0.1 au; {r(narrow,H₂)} ∼ 3± 2 au). The 4.7 μm ¹²CO emission lines are also well fit by two-component profiles ( {{r}broad,CO} =0.4± 0.1 au; {{r}narrow,CO} =15± 2 au). We combine these results with 10 μm observations to form a picture of gapped structure within the mm-imaged dust cavity, providing the first such overview of the inner regions of a young disk. The HST SED of RY Lupi is available online for use in modeling efforts

The Chemistry of Atmosphere-Forest Exchange (CAFE) Model – Part 2: Application to BEARPEX-2007 observations

In a companion paper, we introduced the Chemistry of Atmosphere-Forest Exchange (CAFE) model, a vertically-resolved 1-D chemical transport model designed to probe the details of near-surface reactive gas exchange. Here, we apply CAFE to noontime observations from the 2007 Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX-2007). In this work we evaluate the CAFE modeling approach, demonstrate the significance of in-canopy chemistry for forest-atmosphere exchange and identify key shortcomings in the current understanding of intra-canopy processes. CAFE generally reproduces BEARPEX-2007 observations but requires an enhanced radical recycling mechanism to overcome a factor of 6 underestimate of hydroxyl (OH) concentrations observed during a warm (~29 °C) period. Modeled fluxes of acyl peroxy nitrates (APN) are quite sensitive to gradients in chemical production and loss, demonstrating that chemistry may perturb forest-atmosphere exchange even when the chemical timescale is long relative to the canopy mixing timescale. The model underestimates peroxy acetyl nitrate (PAN) fluxes by 50% and the exchange velocity by nearly a factor of three under warmer conditions, suggesting that near-surface APN sinks are underestimated relative to the sources. Nitric acid typically dominates gross dry N deposition at this site, though other reactive nitrogen (NO_y) species can comprise up to 28% of the N deposition budget under cooler conditions. Upward NO_2 fluxes cause the net above-canopy NO_y flux to be ~30% lower than the gross depositional flux. CAFE under-predicts ozone fluxes and exchange velocities by ~20%. Large uncertainty in the parameterization of cuticular and ground deposition precludes conclusive attribution of non-stomatal fluxes to chemistry or surface uptake. Model-measurement comparisons of vertical concentration gradients for several emitted species suggests that the lower canopy airspace may be only weakly coupled with the upper canopy. Future efforts to model forest-atmosphere exchange will require a more mechanistic understanding of non-stomatal deposition and a more thorough characterization of in-canopy mixing processes