arXiv:1107.3872v1 [astro-ph.SR] 20 Jul 2011 To appear in ApJL, 2011 Preprint typeset using L A T E X style emulateapj v. 11/10/09 A DIM CANDIDATE COMPANION TO ǫ CEPHEI D. Mawet 1 European Southern Observatory, Alonso de Cord´ ova 3107, Vitacura, Santiago, Chile B. Mennesson, E. Serabyn, K. Stapelfeldt Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA O. Absil 2 Institut d’Astrophysique et de G´ eophysique de Li` ege, University of Li` ege, 17 All´ ee du 6 Aoˆ ut, 4000 Sart Tilman, Belgium To appear in ApJL, 2011 ABSTRACT Using a vector vortex coronagraph behind the 1.5-m well-corrected subaperture (WCS) at Palomar, we detected a second object very close to ǫ Cephei, a δ Scuti F0 IV star. The candidate companion, ∼ 50 times fainter than ǫ Cephei, if physically associated, is a late-type K or early M star, and lies at an angular separation of 330 mas, or 1.1 λ/D for the WCS, making it the smallest angle detection ever realized with a coronagraph in terms of λ/D units. The projected separation of the putative companion is ∼ 8.6 AU, most likely on a highly eccentric orbit. The recently detected near-infrared excess is thus likely not due to hot dust. Moreover, we also show that the previously reported IRAS 60 µm excess was due to source confusion on the galactic plane. Subject headings: instrumentation: high angular resolution – instrumentation: adaptive optics – tech- niques: high angular resolution – stars: low-mass 1. INTRODUCTION High contrast imaging at small angles is a very use- ful tool to access inner regions around stars. While in- strumental limits are constantly improved by the devel- opment of exoplanet imaging and characterization tech- niques, such capabilities can be used in stellar astro- physics to discover new systems or put constraint on known ones. Moreover, this small-angle capability allows a significant reduction in the size of potential space tele- scopes aimed at detecting and characterizing exoplanets, which is a cost-effective way of building exoplanet mis- sions. We have used a 1.5-m subaperture of the 5.1-m Hale telescope at Palomar with a small inner working angle (IWA) phase-mask coronagraph to detect a new candi- date stellar companion to the δ Scuti star ǫ Cephei. The result is interesting for two reasons. First, ǫ Cephei was claimed to have an infrared excess measured by IRAS, and recently detected by near-infrared ground-based in- terferometry. Such infrared excesses are generally at- tributed to dusty debris disks (Absil et al. 2006). We show here with an image that the origin of the near- infrared excess is a second object that is likely a late-type stellar companion. Moreover, we demonstrate that the previously reported 60 µm IRAS excess is attributable to an unrelated field object. Second, the potential com- panion was imaged at 1.1 diffraction beamwidths (λ/D, with the working wavelength λ ≃ 2.16 µm, and the tele- scope diameter D =1.5 m) from the star. Such a small angle detection was only possible because of our phase mask vector vortex coronagraph (VVC), demonstrating 1 Jet Propulsion Laboratory, California Institute of Technol- ogy, 4800 Oak Grove Drive, Pasadena, CA 91109, USA 2 Postdoctoral Researcher F.R.S.-FNRS (Belgium) that such coronagraphs can effectively provide very small IWA. 2. THE δ SCUTI ǫ CEPHEI ǫ Cephei is a bright (V =4.2, H =3.7, K =3.5) and nearby (d = 26.2 ± 0.3 pc, see van Leeuwen (2007)) F0 IV star with a temperature T = 7350 K, of the δ Scuti type (Table 1). Privileged targets in asteroseismologic studies (Bruntt et al. 2007), δ Scutis are multi-periodic variable stars lying at the base of the Cepheid instability strip, where it crosses the main sequence. Their small amplitude (0.001–1 mag) variability is due to simultane- ous radial and non-radial pulsation modes with periods between 0.25 and 5 hours. The spectral type of δ Scuti stars ranges from A0 to F5, with nominal mass and tem- perature around 2 M Sun and 7500 K, respectively. For ǫ Cephei, Kennelly et al. (1999) derived a mass of 1.8 ±0.2 M Sun . ǫ Cephei was claimed to be detected by IRAS at 12, 25, and 60 µ as PSC 22132+5647. The reported 60 µ flux density of 1.20 ± 0.08 Jy is > 10 times the ex- pected photospheric value (the ratio of excess infrared luminosity divided by the total energy output from the photosphere was reported to be τ ≃ 1.56 × 10 −4 ). On this basis, Oudmaijer et al. (1992), Moor et al. (2006), and Rhee et al. (2007) identified the system as hosting a bright debris disk. In Rhee et al. (2007), this excess was fitted with a single blackbody giving a putative ring of dust at 62 AU with a temperature of 65 K. Using space velocities UVW, lithium abundance and location on a HR diagram, Rhee et al. (2007) also estimated the age of ǫ Cephei to be a very uncertain ∼ 600 Myr. In the course of an on-going survey for bright exozo- diacal disks around main sequence stars, lead by one of us, the CHARA-FLUOR interferometer detected a