The Sun's position in the sky

We express the position of the Sun in the sky as a function of time and the observer's geographic coordinates. Our method is based on applying rotation matrices to vectors describing points on the celestial sphere. We also derive direct expressions, as functions of date of the year and geographic latitude, for the duration of daylight, the maximum and minimum altitudes of the Sun, and the cardinal directions to sunrise and sunset. We discuss how to account for the eccentricity of the earth's orbit, the precessions of the equinoxes and the perihelion, the size of the solar disk, and atmospheric refraction. We illustrate these results by computing the dates of "Manhattanhenge" (when sunset aligns with the east-west streets on the main traffic grid for Manhattan, in New York City), by plotting the altitude of the Sun over representative cities as a function of time, and by showing plots ("analemmas") for the position of the Sun in the sky at a given hour of the day.Comment: 19 pages, 16 figures. v3: Replaced to match published version and to re-package Mathematica notebook as an ancillary fil

Mid-IR observations of circumstellar disks -- Part III: A mixed sample of PMS stars and Vega-type objects

We present new mid-infrared spectra for a sample of 15 targets (1 FU Orionis object, 4 Herbig Ae stars, 5 T Tauri stars and 5 Vega type stars), obtained with the TIMMI2 camera at La Silla Observatory (ESO). Three targets are members of the beta Pic moving group (HD 155555, HD 181296 and HD 319139). PAH bands are observed towards the T Tauri star HD 34700 and the Herbig Ae star PDS 144 N. For HD 34700, the band profiles indicate processed PAHs. The spectrum of the Vega-type object eta Corvi (HD 109085), for which a resolved disk at sub-mm wavelengths is known, is entirely stellar between 8--13 micron. Similarly, no indication for circumstellar matter at mid-infrared wavelengths is found towards the Vega-like stars HD 3003, HD 80951, HD 181296 and, surprisingly, the T Tauri system HD 155555. The silicate emission features of the remaining eight sources are modelled with a mixture of silicates of different grain sizes and composition. Unprocessed dust dominates FU Ori, HD 143006 and CD-43 344. Large amorphous grains are the main dust component around HD 190073, HD 319139, KK Oph and PDS 144 S. Both small grains and crystalline dust is found for the Vega-type HD 123356, with a dominance of small amorphous grains. We show that the infrared emission of the binary HD 123356 is dominated by its late-type secondary, but optical spectroscopy is still required to confirm the age of the system and the spectral class of the companion. For most targets this is their first mid-infrared spectroscopic observation. We investigate trends between stellar, disk and silicate properties and confirm correlations of previous studies. Several objects present an exciting potential for follow-up high-resolution disk studies.Comment: Accepted for publication in A&

Exocomet signatures around the A-shell star Φ\Phi Leo?

We present an intensive monitoring of high-resolution spectra of the Ca {\sc ii} K line in the A7IV shell star $\Phi$ Leo at very short (minutes, hours), short (night to night), and medium (weeks, months) timescales. The spectra show remarkable variable absorptions on timescales of hours, days, and months. The characteristics of these sporadic events are very similar to most that are observed toward the debris disk host star $\beta$ Pic, which are commonly interpreted as signs of the evaporation of solid, comet-like bodies grazing or falling onto the star. Therefore, our results suggest the presence of solid bodies around $\Phi$ Leo. To our knowledge, with the exception of $\beta$ Pic, our monitoring has the best time resolution at the mentioned timescales for a star with events attributed to exocomets. Assuming the cometary scenario and considering the timescales of our monitoring, our results indicate that $\Phi$ Leo presents the richest environment with comet-like events known to date, second only to $\beta$ Pic.Comment: A&A letters, proof-correcte

Mid-InfraRed imaging of the circumstellar dust around three Herbig Ae stars : HD135344, CQTau, HD163296

Planet formation has been known for many years to be tied to the spatial distribution of gas and dust in disks around young stars. To constrain planet formation models, imaging observations of protoplanetary disks are required. In this framework, we have undertaken a mid-infrared imaging survey of Herbig Ae stars, which are pre-main sequence stars of intermediate mass still surrounded by a large amount of circumstellar material. The observations were made at a wavelength of 20.5 $\mu$m with the CAMIRAS camera mounted at the Cassegrain focus of the Canada France Hawaii Telescope. We report the observations of three stars, HD135344, CQTau and HD163296. The circumstellar material around the three objects is spatially resolved. The extensions feature a disk like shape. The images provide direct information on two key parameters of the disk : its inclination and its outer radius. The outer radius is found to be quite different from the one deduced from disk models only constrained by fitting the Spectral Energy Distribution of the object. Other parameters of the disk, such as flaring, dust mass have been deduced from fitting both the observed extension and the spectral energy distribution with sophisticated disk models. Our results show how important imaging data are to tighten constraints on the disk model parameters.Comment: 10 pages, Accepted in A&

What can we learn about protoplanetary disks from analysis of mid-infrared carbonaceous dust emission?

In this Paper we analyze the mid-infrared (mid-IR) emission of very small dust particles in a sample of 12 protoplanetary disks to see how they are connected to interstellar dust particles and to investigate the possibility that their emission can be used as a probe of the physical conditions and evolution of the disk. We define a basis made of three mid-IR template spectra PAH$^0$, PAH$^+$ and VSGs that were derived from the analysis of reflection nebulae, and an additional PAH$^x$ spectrum that was introduced by Joblin et al. (2008) for the analysis of the spectra of planetary nebulae. From the optimization of the fit of 12 star+disk spectra, using a linear combination of the 4 template spectra, we found that an additional small grain component with a broad feature at 8.3 $\mu$m is needed. We find that the fraction of VSG emission in disks decreases with increasing stellar temperature. VSGs appear to be destroyed by UV photons at the surface of disks, thus releasing free PAH molecules, which are eventually ionized as it is observed in photodissociation regions. On the opposite, we observe that the fraction of PAH$^x$ increases with increasing star temperature except in the case of B stars where they are absent. We argue that this is compatible with the identification of PAH$^x$ as large ionized PAHs, most likely emitting in regions of the disk that are close to the star. Finally, we provide a UV-dependant scheme to explain the evolution of PAHs and VSGs in protoplanetary disks. We show that A stars modify the size spectrum of PAHs and VSGs in favor of large PAHs while B stars destroy even the largest PAHs up to large radii in the disk. These results allow us to put new constrains on the properties of two sources: IRS 48 and "Gomez's Hamburger" which are poorly characterized.Comment: Accepted for publication in A&

Infrared spectroscopy of Nova Cassiopeiae 1993 (V705 Cas). IV. A closer look at the dust

Nova Cassiopeiae 1993 (V705 Cas) was an archetypical dust-forming nova. It displayed a deep minimum in the visual light curve, and spectroscopic evidence for carbon, hydrocarbon and silicate dust. We report the results of fitting the infrared spectral energy distribution with the DUSTY code, which we use to determine the properties and geometry of the emitting dust. The emission is well described as originating in a thin shell whose dust has a carbon:silicate ratio of ~2:1 by number (1.26:1 by mass) and a relatively flat size distribution. The 9.7micron and 18micron silicate features are consistent with freshly-condensed dust and, while the lower limit to the grain size distribution is not well constrained, the largest grains have dimensions \~0.06micron; unless the grains in V705 Cas were anomalously small, the sizes of grains produced in nova eruptions may previously have been overestimated in novae with optically thick dust shells. Laboratory work by Grishko & Duley may provide clues to the apparently unique nature of nova UIR features.Comment: 11 pages, 9 fugure

Searching for a link between the magnetic nature and other observed properties of Herbig Ae/Be stars and stars with debris disks

Among the 21 Herbig Ae/Be stars studied, new detections of a magnetic field were achieved in six stars. For three Herbig Ae/Be stars, we confirm previous magnetic field detections. The largest longitudinal magnetic field, = -454+-42G, was detected in the Herbig Ae/Be star HD101412 using hydrogen lines. No field detection at a significance level of 3sigma was achieved in stars with debris disks. Our study does not indicate any correlation of the strength of the longitudinal magnetic field with disk orientation, disk geometry, or the presence of a companion. We also do not see any simple dependence on the mass-accretion rate. However, it is likely that the range of observed field values qualitatively supports the expectations from magnetospheric accretion models giving support for dipole-like field geometries. Both the magnetic field strength and the X-ray emission show hints for a decline with age in the range of ~2-14Myrs probed by our sample supporting a dynamo mechanism that decays with age. However, our study of rotation does not show any obvious trend of the strength of the longitudinal magnetic field with rotation period. Furthermore, the stars seem to obey the universal power-law relation between magnetic flux and X-ray luminosity established for the Sun and main-sequence active dwarf stars.Comment: 21 pages, 16 figures, 7 tables, accepted for publication in A&

Accretion-related properties of Herbig Ae/Be stars. Comparison with T Tauris

We look for trends relating the mass accretion rate (Macc) and the stellar ages (t), spectral energy distributions (SEDs), and disk masses (Mdisk) for a sample of 38 HAeBe stars, comparing them to analogous correlations found for classical T Tauri stars. Our goal is to shed light on the timescale and physical processes that drive evolution of intermediate-mass pre-main sequence objects. Macc shows a dissipation timescale \tau = 1.3^{+1.0}_{-0.5} Myr from an exponential law fit, while a power law yields Macc(t) \propto t^{-\eta}, with \eta = 1.8^{+1.4}_{-0.7}. This result is based on our whole HAeBe sample (1-6 Msun), but the accretion rate decline most probably depends on smaller stellar mass bins. The near-IR excess is higher and starts at shorter wavelengths (J and H bands) for the strongest accretors. Active and passive disks are roughly divided by 2 x 10^{-7} Msun/yr. The mid-IR excess and the SED shape from the Meeus et al. classification are not correlated with Macc. We find Macc \propto Mdisk^{1.1 +- 0.3}. Most stars in our sample with signs of inner dust dissipation typically show accretion rates ten times lower and disk masses three times smaller than the remaining objects. The trends relating Macc with the near-IR excess and Mdisk extend those for T Tauri stars, and are consistent with viscous disk models. The differences in the inner gas dissipation timescale, and the relative position of the stars with signs of inner dust clearing in the Macc-Mdisk plane, could be suggesting a slightly faster evolution, and that a different process - such as photoevaporation - plays a more relevant role in dissipating disks in the HAeBe regime compared to T Tauri stars. Our conclusions must consider the mismatch between the disk mass estimates from mm fluxes and the disk mass estimates from accretion, which we also find in HAeBe stars.Comment: 11 pages, 7 figures, 1 appendix. Accepted in A&

The young B-star quintuple system HD 155448

Until now, HD 155448 has been known as a post-AGB star and listed as a quadruple system. In this paper, we study the system in depth and reveal that the B component itself is a binary and that the five stars HD 155448 A, B1, B2, C, and D likely form a comoving stellar system. From a spectroscopic analysis we derive the spectral types and find that all components are B dwarfs (A: B1V, B1: B6V, B2: B9V, C: B4Ve, D: B8V). Their stellar ages put them close to the ZAMS, and their distance is estimated to be ~2 kpc. Of particular interest is the C component, which shows strong hydrogen and forbidden emission lines at optical wavelengths. All emission lines are spatially extended in the eastern direction and appear to have a similar velocity shift, except for the [OI] line. In the IR images, we see an arc-like shape to the northeast of HD 155448 C. From the optical up to 10 micron, most circumstellar emission is located at distances between ~1.0 arcsec and 3.0 arcsec from HD 155448 C, while in the Q band the arc-like structure appears to be in contact with HD 155448 C. The Spitzer and VLT/VISIR mid-IR spectra show that the circumstellar material closest to the star consists of silicates, while polycyclic aromatic hydrocarbons (PAH) dominate the emission at distances >1 arcsec with bands at 8.6, 11.3, and 12.7 micron. We consider several scenarios to explain the unusual, asymmetric, arc-shaped geometry of the circumstellar matter. The most likely explanation is an outflow colliding with remnant matter from the star formation process.Comment: 19 pages, 12 figures, 9 tables. Accepted for publication in A&

An emission ring at 20 microns around the HAEBE star AB Aurigae: unveiling the disc structure

Isolated HAEBE stars are believed to represent an intermediate stage of objects between young stellar objects surrounded by massive, optically thick, gaseous and dusty disks and Vega like stars surrounded by debris disks. The star AB Aur is already known for being surrounded by an intermediate-stage dust disk emitting a fairly large infrared and (sub-)millimetric excess. Until now, the outer disk structure has only been resolved at millimeter wavelengths and at optical wavelength coronographic imaging. We have obtained 20 microns images which show an unexpected ellipse-shaped disk structure in emission at a distance of about 260 AU from the central star. Large azimuthal asymmetries in brightness can be noticed and the center of the ellipse does not coincide with the star. A simple, pure geometrical model based on an emission ring of uniform surface brightness, but having an intrinsic eccentricity succeeds in fitting the observations. These observations give for the first time clues on a very peculiar structure of pre-main-sequence disk geometry, i.e. a non uniform increase in the disk thickness unlike the common usual sketch of a disk with a constant flaring angle. They provide also valuable informations on the disk inclination as well as its dust composition; at such a large distance from the star, only transient heating of very small particles can explain such a bright ring of emission at mid-infrared wavelengths. Finally, the increase of thickness inferred by the model could be caused by disk instabilities; the intrinsic eccentricity of the structure might be a clue to the presence of a massive body undetected yet