The enigmatic young object: Walker 90/V590 Monocerotis ^*,**

¹ Los Alamos National Laboratory, PO Box 1663, ISR-1, MS B244, Los Alamos, NM 87545, USA e-mail: mperez@lanl.gov
² Caltech, SIRTF Science Center, MS, 314-6, Pasadena, CA 91125, USA e-mail: mccollum@ipac.caltech.edu
³ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748, Garching bei München, Germany e-mail: mvandena@eso.org
⁴ Brigham Young University, Dept. of Physics and Astronomy – ESC – N488, Provo, Utah 84602, USA e-mail: jonerm@forty-two.byu.edu

Received: 8 April 2008
Accepted: 17 May 2008

Abstract

Aims. We assess the evolutionary status of the intriguing object Walker 90/V590 Mon, which is located about 20 arcmin northwest of the Cone Nebula near the center of the open cluster NGC 2264. This object, according to its most recent optical spectral type determination (B7), which we confirmed, is at least 3 mag too faint in V for the cluster distance, but it shows the classical signs of a young pre-main sequence object, such as highly variable Hα emission, Mg II emission, IR excess, UV continuum, and optical variability.

Methods. We analyzed a collection of archival and original data on Walker 90, covering 45 years including photometry, imaging, and spectroscopic data ranging from ultraviolet to near-infrared wavelengths.

Results. According to star formation processes, it is expected that, as this object clears its primordial surroundings, it should become optically brighter, show a weakening of its IR excess and present decreasing line emissions. This behavior is supported by our observations and analysis, but timescales are expected to be longer than the one observed here. Based on photometric data secured in 2007, we find Walker 90 at its brightest recorded optical magnitude . We document an evolution in spectral type over the past five decades (from A2/A3 to currently B7 and as early as B4), along with a decrease in the near-infrared K fluxes. From near-infrared VISIR  images secured in 2004, Walker 90 appears as a point source placing an upper limit of < 0.1″ for its diameter. Evidence of turbulent inflows is found in rapidly changing inverse P-Cygni profiles in the lower Balmer lines, with a broadening of ±400 km s^-1 in Hα and a redshifted component in Hβ with a terminal velocity of ~600 km s^-1. The measured steep UV continuum fluxes (mimicking a star as early as B4), added to a tentative identification of N V emission, suggest a strong non-photospheric component, typically of fluxes arising from a thermally inhomogeneous accretion disk. We detect a well defined 2200 Å bump, indicative of dense material in the line-of-sight. We conclude that many observational features are explained if W90 is a flared disk system, surrounded by an inclined optically thick accretion disk.