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International audience; In this white paper, we explore how higher angular resolution beyond ALMA and 8m-class telescopes can extend our understanding of the key stages of planet formation, to resolve accreting circumplanetary disks themselves, and to watch planets forming in situ for the nearest star-forming regions.

Context. There is still a debate about the nature of the mechanism that causes the pulsation excitation of the rapidly oscillating Ap stars that oscillate above the highest theoretically acoustic frequency. HD 24712 is a good test case for such a study because it is bright, its parallax accurately determined, and its frequency spectrum is well known. Aims. Visible long-baseline interferometry is a unique technique for measuring accurate angular diameters of targets as small as the brightest roAp stars, and thus estimating accurate radii by a method as independent as possible of atmosphere models. Methods. We used the visible spectrograph VEGA at the CHARA long-baseline optical array to observe HD 24712, and we derived its limb-darkened diameter. We also estimated its bolometric flux from spectroscopic data in the literature and determined its radius, luminosity, and effective temperature. Results. We determined a limb-darkened angular diameter of 0.335 ± 0.009 mas for HD 24712 and derived a radius of R = 1.772 ± 0.057 R⊙, a luminosity of L = 7.2 ± 1.8 L⊙, and an effective temperature of Teff = 7235 ± 280 K, which is in very close agreement with the values provided by the self-consistent stratified model developed for this star. We used these fundamental parameters to set HD 24712 in the Hertzsprung-Russell diagram. Its position is marginally consistent with the region where high radial order modes are predicted to be excited by the κ-mechanism. Conclusions. We conclude that oscillations in this star are most likely not driven by the κ-mechanism. peerReviewed

The bright star 55 Cancri is known to host five planets, including a transiting super-Earth. We use the CHARA Array to directly determine the following of 55 Cnc's stellar astrophysical parameters: $R=0.943 \pm 0.010 R_{\odot}$, $T_{\rm EFF} = 5196 \pm 24$ K. Planet 55 Cnc f ($M \sin i = 0.155 M_{Jupiter}$) spends the majority of the duration of its elliptical orbit in the circumstellar habitable zone (0.67--1.32 AU) where, with moderate greenhouse heating, it could harbor liquid water. Our determination of 55 Cancri's stellar radius allows for a model-independent calculation of the physical diameter of the transiting super-Earth 55 Cnc e ($\simeq 2.1 R_{\earth}$), which, depending on the assumed literature value of planetary mass, implies a bulk density of 0.76 $\rho_{\earth}$ or 1.07 $\rho_{\earth}$., Comment: 2 pages, 1 figure, 1 table. To appear in "Transiting Planets, Vibrating Stars, and Their Connection", Conference Proceedings of the 2nd CoRoT Symposium, Eds: A. Baglin, M. Deleuil, E. Michel, C. Moutou

We have obtained H-band interferometric observations of three galactic red supergiant stars using the MIRC instrument on the CHARA array. The targets include AZ Cyg, a field RSG and two members of the Per OB1 association, RS Per and T Per. We find evidence of departures from circular symmetry in all cases, which can be modelled with the presence of hotspots. This is the first detection of these features in the $H$-band. The measured mean diameters and the spectral energy distributions were combined to determine effective temperatures. The results give further support to the recently derived hotter temperature scale of red supergiant stars by Levesque et al. (2005), which has been evoked to reconcile the empirically determined physical parameters and stellar evolutionary theories. We see a possible correlation between spottedness and mid-IR emission of the circumstellar dust, suggesting a connection between mass-loss and the mechanism that generates the spots., 6 pages, 5 figures, to appear in the Proc. of "Hot and Cool: Bridging Gaps in Massive Star Evolution", NASA Herschel Science Center, Nov 10-12, 2008

We have been using the CHARA Interferometric Array on Mount Wilson over the course of the last 4 years to directly determine diameters of stars. When coupled with literature photometry, these radii provide stellar effective temperatures, luminosities, and thus habitable zone locations. This presentation reviews recent results of our survey, particularly with respect to the stars that host extrasolar planets around them, and what we have learned about the planets from studying the parent stars.

We present measurements of fundamental astrophysical properties of nearby, low-mass, K- and M-dwarfs from our DISCOS survey (DIameterS of COol Stars). The principal goal of our study is the determination of linear radii and effective temperatures for these stars. We calculate their radii from angular diameter measurements using the CHARA Array and Hipparcos distances. Combined with bolometric flux measurements based on literature photometry, we use our angular diameter results to calculate their effective surface temperatures. We present preliminary results established on an assortment of empirical relations to the stellar effective temperature and radius that are based upon these measurements. We elaborate on the discrepancy seen between theoretical and observed stellar radii, previously claimed to be related to stellar activity and/or metallicity. Our preliminary conclusion, however, is that convection plays a larger role in the determination of radii of these late-type stars. Understanding the source of the radius disagreement is likely to impact other areas of study for low-mass stars, such as the detection and characterization of extrasolar planets in the habitable zones., Comment: Contribution to Proceedings of Cool Stars 16 Workshop; 8 pages in ASP format; 9 figures

International audience; GJ 876 is a nearby M dwarf hosting four currently known extrasolar planets. We use the CHARA interferometric array to determine the stellar angular diameter. Coupled with trigonometric parallax values and literature photometry, we obtain direct estimates of the stellar physical size, surface temperature, luminosity, and location / extent of the system's habitable zone (HZ). We present our measurement results of the GJ 876 stellar astrophysical parameters and the location of the HZ relative to the planetary orbits.

Over 300 extrasolar planets (exoplanets) have been detected orbiting nearby stars. We now hope to conduct a census of all planets around nearby stars and to characterize their atmospheres and surfaces with spectroscopy. Rocky planets within their star's habitable zones have the highest priority, as these have the potential to harbor life. Our science goal is to find and characterize all nearby exoplanets; this requires that we measure the mass, orbit, and spectroscopic signature of each one at visible and infrared wavelengths. The techniques for doing this are at hand today. Within the decade we could answer long-standing questions about the evolution and nature of other planetary systems, and we could search for clues as to whether life exists elsewhere in our galactic neighborhood., Comment: 7 pages, 2 figures, submitted to Astro2010 Decadal Review