The Hidden Clumps in VY CMa Uncovered by ALMA
arxiv(2024)
摘要
The red hypergiant VY CMa is famous for its very visible record of high mass
loss events. Recent CO observations with ALMA revealed three previously unknown
large scale outflows (Paper I). In this paper we use the CO maps to investigate
the motions of a cluster of four clumps close to the star, not visible in the
optical or infrared images. We present their proper motions measured from two
epochs of ALMA images and determine the line of sight velocities of the gas in
emission at the clumps. We estimate their masses and ages, or time since
ejection, and conclude that all four were ejected during VY CMa's active period
in the early 20th century. Together with two additional knots observed with
HST, VY CMa experienced at least six massive outflows during a 30 year period
with a total mass lost greater than 0.07 Msun. The position-velocity map of the
^12CO emission reveals previously unnoticed attributes of the older outer
ejecta. In a very narrow range of Doppler velocities, ^12CO absorption and
emission causes some of this outer material to be quite opaque. At those
frequencies the inner structure is hidden and we see only emission from an
extended outer region. This fact produces a conspicuous but illusory dark spot
if one attempts to subtract the continuum in a normal way.
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