Rotating and infalling motion around the high-mass young stellar object Cepheus A-HW2 observed with the methanol maser at 6.7 GHz ^⋆

¹ Graduate school of Science and Engineering, Yamaguchi University, 1677-1 Yoshida, Yamaguchi 753-8512, Japan
e-mail: koichiro@yamaguchi-u.ac.jp; niinuma@yamaguchi-u.ac.jp
² Research Institute for Time Studies, Yamaguchi University, 1677-1 Yoshida, Yamaguchi 753-8511, Japan
e-mail: kenta@yamaguchi-u.ac.jp; motogi@yamaguchi-u.ac.jp
³ The Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuou-ku, Sagamihara 252-5210, Japan
e-mail: akihiro.doi@vsop.isas.jaxa.jp; murata@vsop.isas.jaxa.jp
⁴ Department of Space and Astronautical Science, The Graduate University for Advanced Studies, 3-1-1 Yoshinodai, Chuou-ku, Sagamihara 252-5210, Japan
⁵ VERA Project, National Astronomical Observatory of Japan (NAOJ), 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
e-mail: mareki.honma@nao.ac.jp; hideyuki.kobayashi@nao.ac.jp
⁶ Department of Astronomical Science, Graduate University for Advanced Studies, 2-21-1 Osawa, Mitaka 181-8588, Japan
⁷ Department of Physical Science, School of Science, Osaka Prefecture University, 1-1 Gakuen-cho, Nakaku, Sakai 599-8531, Japan
e-mail: ogawa@p.s.osakafu-u.ac.jp
⁸ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, 200030 Shanghai, PR China
e-mail: kwajima@shao.ac.cn
⁹ Mizusawa VLBI Observatory, NAOJ, 2-12 Hoshigaoka-cho, Mizusawa-ku, Oshu 023-0861, Japan
e-mail: satoko.ss@nao.ac.jp
¹⁰ School of Mathematics and Physics, University of Tasmania, Private Bag 37, Hobart TAS7001, Australia
e-mail: Simon.Ellingsen@utas.edu.au

Received: 12 February 2013
Accepted: 3 December 2013

Abstract

Context. Proper motion observations of masers can provide information on dynamic motions on scales of a few milliarcseconds per year (mas yr^-1) at radii of 100–1000 au scales from central young stellar objects (YSOs).

Aims. The 6.7 GHz methanol masers are one of the best probes for investigations of the dynamics of high-mass YSOs, and in particular for tracing the rotating disk. We have measured the internal proper motions of the 6.7 GHz methanol masers associated with Cepheus A (Cep A) HW2 using Very Long Baseline Interferometery (VLBI) observations.

Methods. We conducted three epochs of VLBI monitoring observations of the 6.7 GHz methanol masers in Cep A-HW2 with the Japanese VLBI Network (JVN) over the period 2006–2008. In 2006, we were able to use phase-referencing to measure the absolute coordinates of the maser emission with an accuracy of a few milliarcseconds. We compared the maser distribution with other molecular line observations that trace the rotating disk.

Results. We measured the internal proper motions for 29 methanol maser spots, of which 19 were identified at all three epochs and the remaining ten at only two epochs. The magnitude of proper motions ranged from 0.2 to 7.4 km s^-1, with an average of 3.1 km s^-1. Although there are large uncertainties in the observed internal proper motions of the methanol maser spots in Cep A, they are well fitted by a disk that includes both rotation and infall velocity components. The derived rotation and infall velocities at the disk radius of 680 au are 0.5 ± 0.7 and 1.8 ± 0.7 km s^-1, respectively.

Conclusions. Assuming that the modeled disk motion accurately represents the accretion disk around the Cep A-HW2 high-mass YSO, we estimated the mass infall rate to be 3 × 10^-4  n₈  M_⊙  yr^-1 (n₈ is the gas volume density in units of 10⁸  cm^-3). The combination of the estimated mass infall rate and the magnitude of the fitted infall velocity suggests that Cep A-HW2 is at an evolutionary phase of active gas accretion from the disk onto the central high-mass YSO. The infall momentum rate is estimated to be 5 × 10^-4  n₈  M_⊙  yr^-1  km  s^-1, which is larger than the estimated stellar radiation pressure of the HW2 object, supporting the hypothesis that this object is in an active gas accretion phase.