An M-dwarf star in the transition disk of Herbig HD 142527 - Physical parameters and orbital elements

A&A 590, A90 (2016) DOI: 10.1051/0004-6361/201527863 Astronomy & Astrophysics c ESO 2016 An M-dwarf star in the transition disk of Herbig HD 142527 Physical parameters and orbital elements S. Lacour1, 2 , B. Biller3 , A. Cheetham4 , A. Greenbaum5 , T. Pearce6 , S. Marino6 , P. Tuthill7 , L. Pueyo8 , E. E. Mamajek9 , J. H. Girard10 , A. Sivaramakrishnan8 , M. Bonnefoy11 , I. Baraffe9, 12 , G. Chauvin11 , J. Olofsson13, 14, 15 , A. Juhasz6 , M. Benisty11 , J.-U. Pott13 , A. Sicilia-Aguilar16 , T. Henning13 , A. Cardwell17 , S. Goodsell18, 19 , J. R. Graham20 , P. Hibon10, 17 , P. Ingraham21 , Q. Konopacky22 , B. Macintosh23 , R. Oppenheimer24 , M. Perrin8 , F. Rantakyrö17 , N. Sadakuni25 , and S. Thomas21 (Affiliations can be found after the references) Received 30 November 2015 / Accepted 25 April 2016 ABSTRACT Aims. HD 42527A is one of the most studied Herbig Ae/Be stars with a transitional disk, as it has the largest imaged gap in any protoplanetary disk: the gas is cleared from 30 to 90 AU. The HD 142527 system is also unique in that it has a stellar companion with a small mass compared to the mass of the primary star. This factor of ≈20 in mass ratio between the two objects makes this binary system different from any other YSO. The HD 142527 system could therefore provide a valuable test bed for understanding the impact of a lower mass companion on disk structure. This low-mass stellar object may be responsible for both the gap and dust trapping observed by ALMA at longer distances. Methods. We observed this system with the NACO and GPI instruments using the aperture masking technique. Aperture masking is ideal for providing high dynamic range even at very small angular separations. We present the spectral energy distribution (SED) for HD 142527A and B. Brightness of the companion is now known from the R band up to the M 0 band. We also followed the orbital motion of HD 142527B over a period of more than two years. Results. The SED of the companion is compatible with a T = 3000 ± 100 K object in addition to a 1700 K blackbody environment (likely a circum-secondary disk). From evolution models, we find that it is compatible with an object of mass 0.13 ± 0.03 M , radius 0.90 ± 0.15 R , and age 1.0+1.0 −0.75 Myr. This age is significantly younger than the age previously estimated for HD 142527A. Computations to constrain the orbital parameters found a semimajor axis of 140+120 −70 mas, an eccentricity of 0.5 ± 0.2, an inclination of 125 ± 15 degrees, and a position angle of the right ascending node of −5 ± 40 degrees. Inclination and position angle of the ascending node are in agreement with an orbit coplanar with the inner disk, not coplanar with the outer disk. Despite its high eccentricity, it is unlikely that HD 142527B is responsible for truncating the inner edge of the outer disk. Key words. protoplanetary disks – planet-disk interactions – binaries: visual – stars: variables: T Tauri, Herbig Ae/Be 1. Introduction During the process of planet formation and disk dissipation, primordial disks begin in an optically thick state with significant emission at infrared wavelengths. As the disks clear out, they pass through an intermediate transitional stage marked by a drop in near- or mid-infrared (IR) emission that indicates the presence of an annular disk gap. These gaps can form through several mechanisms, such as photoevaporation, truncation due to a binary companion, or the presence of a forming planet. This last pathway has led to transitional disks being the subject of close study, as they may provide valuable insights into the planet formation process. HD 142527A, a young stellar object (YSO), is one of the most studied Herbig Ae/Be stars with a transitional disk. With dust and some gases cleared from 30 to 90 AU, it has the largest imaged gap in any protoplanetary disk. According to Mendigutía et al. (2014), the age, distance, and mass for this star are 5 ± 1.5 Myr, 140 ± 20 pc and 2.0 ± 0.3 M , respectively. Biller et al. (2012) discovered a low-mass stellar companion (∼0.2 M ) at ≈12 AU from the star within the gap. This companion was confirmed in the R band by Close et al. (2014). Eccentric orbit or not, it may have played a vital role in carving the large gap in this system. HD 142527A is also notable for its diversity of disk structure. Recent imaging with the Atacama Large Millimeter Array (ALMA) of the outer disk (Casassus et al. 2013, 2015b) reveals large asymmetrical structures composed of millimeter-size grains (similar to the rare horseshoe structure also seen around the young Herbig star WLY 2-48 by van der Marel et al. 2013). These structures are thought to be signposts of density variations possibly caused by planetary formation (Williams & Cieza 2011). Scattering of the stellar light in the near-infrared (NIR) also reveals structures at the surface of the outer disk (Avenhaus et al. 2014). The HD 142527 system provides a valuable test bed for understanding the impact of a lower mass companion on disk structure. As opposed to other young stellar binaries, which generally have nearly equal masses, the HD 142527 system has a more extreme mass ratio (a factor of ∼20). Thus, we can study in situ the effects of this companion, which may be responsible for both the gap and dust trapping observed further out in this disk. However, initial constraints on the mass and other properties of the companion are still vague and at its present apparent separation it does not appear to be responsible for clearing the large disk gap. Many studies have invoked the possibility of forming giant planets on wide orbits exterior to the companion, making HD 142527 Article published by EDP Sciences A90, page 1 of 8 A&A 590, A90 (2016) an exciting target for direct imaging observations. Constraining the mass and orbital parameters of the low-mass companion is vital to determining the origin of the observed disk gap. We have been conducting ongoing orbital monitoring to constrain the companion properties. This paper reports multiple aperture masking observations of HD142527B. The interferometric nature of the technique permits precise measurements of the contrast ratio and separation, even though the companion lies at the diffraction limit of the telescope. In Sect. 2, we present the new sparse aperture masking (SAM) observations. In Sect. 3, we review the basic parameters of HD 142527A. In Sect. 4, we report and discuss the spectral energy distribution (SED) of the companion from visible to mid-infrared (5 µm) light. In Sect. 5, we present the orbital parameters of the companion. In Sect. 6, we discuss the results. 2. Observations and data reduction 2.1. Principle Visibilities (Amplitude) Visibilities (Phases) 5 5 0 0 −5 −5 |V| = 0.85 0.84 |V| = 0.9 |V| = 0.95 −5 0 5 0.88 0.92 0.96 |V| = 1 1.00 −1.84 −1 deg −0.5 deg 0.5 deg −5 0 5 −0.92 0.00 0.92 1 deg 1.84 Fig. 1. SAM K band dataset. The phase and amplitude in the Fourier domain a (...truncated)