79 research outputs found
Multipolar Planetary Nebulae: Not as Geometrically Diversified as Thought
Planetary nebulae (PNe) have diverse morphological shapes, including
point-symmetric and multipolar structures. Many PNe also have complicated
internal structures such as torus, lobes, knots, and ansae. A complete
accounting of all the morphological structures through physical models is
difficult. A first step toward such an understanding is to derive the true
three-dimensional structure of the nebulae. In this paper, we show that a
multipolar nebula with three pairs of lobes can explain many of such features,
if orientation and sensitivity effects are taken into account. Using only six
parameters - the inclination and position angles of each pair - we are able to
simulate the observed images of 20 PNe with complex structures. We suggest that
the multipolar structure is an intrinsic structure of PNe and the statistics of
multipolar PNe has been severely underestimated in the past.Comment: 36 pages, 5 figures, 2 table
Understanding Accretion Outbursts in Massive Protostars through Maser Imaging
The bright maser emission produced by several molecular species at centimeter
to long millimeter wavelengths provides an essential tool for understanding the
process of massive star formation. Unimpeded by the high dust optical depths
that affect shorter wavelength observations, the high brightness temperature of
these emission lines offers a way to resolve accretion and outflow motions down
to scales below 1 au in deeply embedded Galactic star-forming regions at
kiloparsec distances. The recent identification of extraordinary accretion
outbursts in two high-mass protostars, both of which were heralded by maser
flares, has rapidly impacted the traditional view of massive protostellar
evolution, leading to new hydrodynamic simulations that can produce such
episodic outbursts. In order to understand how these massive protostars evolve
in response to such events, larger, more sensitive ground-based centimeter
wavelength interferometers are needed that can simultaneously image multiple
maser species in the molecular gas along with faint continuum from the central
ionized gas. Fiducial observations of a large sample of massive protostars will
be essential in order to pinpoint the progenitors of future accretion
outbursts, and to quantify the outburst-induced changes in their protostellar
photospheres and outflow and accretion structures. Knowledge gained from these
studies will have broader impact on the general topic of accretion onto massive
objects.Comment: Science white paper submitted to the Astro2020 Decadal Survey. arXiv
admin note: substantial text overlap with arXiv:1806.0698
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