199 research outputs found
VLT near- to mid-IR imaging and spectroscopy of the M17 UC1-IRS5 region
We investigate the surroundings of the hypercompact HII region M17 UC1 to
probe the physical properties of the associated young stellar objects and the
environment of massive star formation. Five of the seven point sources in this
region show -band excess emission. Geometric match is found between the H_2
emission and near-IR polarized light in the vicinity of IRS5A, and between the
diffuse mid-IR emission and near-IR polarization north of UC1. The H_2 emission
is typical for dense PDRs, which are FUV pumped initially and repopulated by
collisional de-excitation. The spectral types of IRS5A and B273A are B3-B7
V/III and G4-G5 III, respectively. The observed infrared luminosity L_IR in the
range 1-20 micron is derived for three objects; we obtain 2.0x10^3 L_\sun for
IRS5A, 13 L_\sun for IRS5C, and 10 L_\sun for B273A. IRS5 might be a young
quadruple system. Its primary star IRS5A is confirmed to be a high-mass
protostellar object (~ 9 M_\sun, ~1x10^5 yrs); it might have terminated
accretion due to the feedback from the stellar activities (radiation pressure,
outflow) and the expanding HII region of M17. UC1 might also have terminated
accretion because of the expanding hypercompact HII region ionized by itself.
The disk clearing process of the low-mass YSOs in this region might be
accelerated by the expanding HII region. The outflows driven by UC1 are running
in south-north with its northeastern side suppressed by the expanding
ionization front of M17; the blue-shifted outflow lobe of IRS5A is seen in two
types of tracers along the same line of sight in the form of H_2 emission
filament and mid-emission. The H_2 line ratios probe the properties of M17 SW
PDR, which is confirmed to have a clumpy structure with two temperature
distributions: warm, dense molecular clumps with n_H>10^5 cm^-3 and T~575 K and
cooler atomic gas with n_H~3.7x10^3-1.5x10^4 cm-3 and T~50-200 K.Comment: accepted for publication in A&A, 19 pages, 15 figures, 5 table
CEN34 -- High-Mass YSO in M17 or Background Post-AGB Star?
We investigate the proposed high-mass young stellar object (YSO) candidate
CEN34, thought to be associated with the star forming region M17. Its optical
to near-infrared (550-2500 nm) spectrum reveals several photospheric absorption
features, such as H{\alpha}, Ca triplet and CO bandheads but lacks any emission
lines. The spectral features in the range 8375-8770{\AA} are used to constrain
an effective temperature of 5250\pm250 (early-/mid-G) and a surface gravity of
2.0\pm0.3 (supergiant). The spectral energy distribution of CEN34 resembles the
SED of a high-mass YSO or an evolved star. Moreover, the observed temperature
and surface gravity are identical for high-mass YSOs and evolved stars. The
radial velocity relative to LSR (V_LSR) of CEN34 as obtained from various
photospheric lines is of the order of -60 km/s and thus distinct from the +25
km/s found for several OB stars in the cluster and for the associated molecular
cloud. The SED modeling yields ~ 10^{-4} M_sun of circumstellar material which
contributes only a tiny fraction to the total visual extinction (11 mag). In
the case of a YSO, a dynamical ejection process is proposed to explain the
V_LSR difference between CEN34 and M17. Additionally, to match the temperature
and luminosity, we speculate that CEN34 had accumulated the bulk of its mass
with accretion rate > 4x10^{-3} M_sun/yr in a very short time span (~ 10^3
yrs), and currently undergoes a phase of gravitational contraction without any
further mass gain. However, all the aforementioned characteristics of CEN34 are
compatible with an evolved star of 5-7 M_sun and an age of 50-100 Myrs, most
likely a background post-AGB star with a distance between 2.0 kpc and 4.5 kpc.
We consider the latter classification as the more likely interpretation.
Further discrimination between the two possible scenarios should come from the
more strict confinement of CEN34's distance.Comment: 8 pages, 8 figures, 2 tables; accepted by A&
No-Reference Quality Assessment for 360-degree Images by Analysis of Multi-frequency Information and Local-global Naturalness
360-degree/omnidirectional images (OIs) have achieved remarkable attentions
due to the increasing applications of virtual reality (VR). Compared to
conventional 2D images, OIs can provide more immersive experience to consumers,
benefitting from the higher resolution and plentiful field of views (FoVs).
Moreover, observing OIs is usually in the head mounted display (HMD) without
references. Therefore, an efficient blind quality assessment method, which is
specifically designed for 360-degree images, is urgently desired. In this
paper, motivated by the characteristics of the human visual system (HVS) and
the viewing process of VR visual contents, we propose a novel and effective
no-reference omnidirectional image quality assessment (NR OIQA) algorithm by
Multi-Frequency Information and Local-Global Naturalness (MFILGN).
Specifically, inspired by the frequency-dependent property of visual cortex, we
first decompose the projected equirectangular projection (ERP) maps into
wavelet subbands. Then, the entropy intensities of low and high frequency
subbands are exploited to measure the multi-frequency information of OIs.
Besides, except for considering the global naturalness of ERP maps, owing to
the browsed FoVs, we extract the natural scene statistics features from each
viewport image as the measure of local naturalness. With the proposed
multi-frequency information measurement and local-global naturalness
measurement, we utilize support vector regression as the final image quality
regressor to train the quality evaluation model from visual quality-related
features to human ratings. To our knowledge, the proposed model is the first
no-reference quality assessment method for 360-degreee images that combines
multi-frequency information and image naturalness. Experimental results on two
publicly available OIQA databases demonstrate that our proposed MFILGN
outperforms state-of-the-art approaches
A Chemical Study of Nine Star-forming Regions with Evidence of Infall Motion
The study of the physical and chemical properties of gas infall motion in the
molecular clumps helps us understand the initial stages of star formation. We
used the FTS wide-sideband mode of the IRAM 30-m telescope to observe nine
infall sources with significant double peaked blue line profile. The
observation frequency range are 83.7 - 91.5 GHz and 99.4 - 107.2 GHz. We have
obtained numbers of molecular line data. Using XCLASS, a total of 7 to 27
different molecules and isotopic transition lines have been identified in these
nine sources, including carbon chain molecules such as CCH, c-C3H2 and HC3N.
According to the radiation transfer model, we estimated the rotation
temperatures and column densities of these sources. Chemical simulations
adopting a physical model of HMSFRs are used to fit the observed molecular
abundances. The comparison shows that most sources are in the early HMPO stage,
with the inner temperature around several ten K
Large-scale Patterning of Hydrophobic Silicon Nanostructure Arrays Fabricated by Dual Lithography and Deep Reactive Ion Etching
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