The distance and size of the red hypergiant NML Cygni from VLBA and VLA astrometry

¹ Max-Plank-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: bzhang@mpifr.de
² Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, 200030 Shanghai, PR China
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁴ Department of Astronomy, Nanjing University, 22 Hankou Road, 210093 Nanjing, PR China
⁵ National Radio Astronomy Observatory, Socorro, NM 87801, USA

Received: 11 May 2012
Accepted: 4 July 2012

Abstract

Context. The red hypergiant NML Cyg has been assumed to be at part of the Cyg OB2 association, although its distance has never been measured directly. A reliable distance is crucial to study the properties of this prominent star. For example, its luminosity, and hence its position on the H-R diagram, is critical information to determine its evolutionary status. In addition, a detection of the radio photosphere would be complementary to other methods of determining the stellar size.

Aims. We aim to understand the characteristics of NML Cyg with direct measurements of its absolute position, distance, kinematics, and size.

Methods. We observe circumstellar 22 GHz H₂O and 43 GHz SiO masers with the Very Long Baseline Array to determine the parallax and proper motion of NML Cyg. We observe continuum emission at 43 GHz from the radio photosphere of NML Cyg with the Very Large Array.

Results. We measure the annual parallax of NML Cyg to be 0.620 ±  0.047 mas, corresponding to a distance of 1.61_-0.11^+0.13 kpc. The measured proper motion of NML Cyg is μ_x = −1.55 ± 0.42 mas yr^-1 eastward and μ_y = −4.59 ± 0.41 mas yr^-1 northward. Both the distance and proper motion are consistent with that of Cyg OB2, within their joint uncertainty, confirming their association. Taking into consideration molecular absorption signatures seen toward NML Cyg, we suggest that NML Cyg lies on the far side of the Cyg OB2 association. The stellar luminosity revised with our distance brings NML Cyg significantly below the empirical luminosity limit for a red supergiant. We partially resolve the radio photosphere of NML Cyg at 43 GHz and find its diameter is about 44 mas, suggesting an optical stellar diameter of 22 mas, if the size of radio photosphere is 2 times the optical photosphere. Based on the position of circumstellar SiO masers relative to the radio photosphere, we estimate the absolute position of NML Cyg at epoch 2008.868 to be α_J2000 = 20^h46^m25.^s5382 ±  0.^s0010, δ_J2000 = 40°06′59.′′379 ±  0.′′015. The peculiar motions of NML Cyg, the average of stars in Cyg OB2, and four other star-forming regions rules out that an expanding “Strömgren sphere” centered on Cyg OB2 is responsible for the kinematics of the Cygnus X region.