اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدStochastic grain heating and mid-infrared emission in protostellar cores
186
0
0.0
(
0
)
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
Stochastic heating of small grains is often mentioned as a primary cause of large infrared (IR) fluxes from star-forming galaxies, e.g. at 24mu m. If the mechanism does work at a galaxy-wide scale, it should show up at smaller scales as well. We calculate temperature probability density distributions within a model protostellar core for four dust components: large silicate and graphite grains, small graphite grains, and polycyclic aromatic hydrocarbon particles. The corresponding spectral energy distributions are calculated and compared with observations of a representative infrared dark cloud core. We show that stochastic heating, induced by the standard interstellar radiation field, cannot explain high mid-IR emission toward the centre of the core. In order to reproduce the observed emission from the core projected centre, in particular, at 24mu m, we need to increase the ambient radiation field by a factor of about 70. However, the model with enhanced radiation field predicts even higher intensities at the core periphery, giving it a ring-like appearance, that is not observed. We discuss possible implications of this finding and also discuss a role of other non-radiative dust heating processes.
قيم البحث
اقرأ أيضاً
In recent years, dramatic outbursts have been identified toward massive protostars via infrared and millimeter dust continuum and molecular maser emission. The longest lived outburst ($>6$ yr) persists in NGC6334I-MM1, a deeply-embedded object with n
o near-IR counterpart. Using FORCAST and HAWC+ on SOFIA, we have obtained the first mid-infrared images of this field since the outburst began. Despite being undetected in pre-outburst ground-based 18 $mu$m images, MM1 is now the brightest region at all three wavelengths (25, 37, and 53 $mu$m), exceeding the ultracompact HII region MM3 (NGC6334F). Combining the SOFIA data with ALMA imaging at four wavelengths, we construct a spectral energy distribution of the combination of MM1 and the nearby hot core MM2. The best-fit Robitaille radiative transfer model yields a luminosity of $(4.9pm0.8)times10^4 L_odot$. Accounting for an estimated pre-outburst luminosity ratio MM1:MM2 = $2.1pm0.4$, the luminosity of MM1 has increased by a factor of $16.3pm4.4$. The pre-outburst luminosity implies a protostar of mass 6.7 $M_odot$, which can produce the ionizing photon rate required to power the pre-outburst hypercompact HII region surrounding the likely outbursting protostar MM1B. The total energy and duration of the outburst exceed the S255IR-NIRS3 outburst by a factor of $gtrsim3$, suggesting a different scale of event involving expansion of the protostellar photosphere (to $gtrsim$ 20 $R_odot$), thereby supporting a higher accretion rate ($gtrsim$0.0023 $M_odot$ yr$^{-1}$) and reducing the ionizing photon rate. In the grid of hydrodynamic models of Meyer et al. 2021, the combination of outburst luminosity and magnitude (3) places the NGC6334I-MM1 event in the region of moderate total accretion ($sim$0.1-0.3 $M_odot$) and hence long duration ($sim$40-130 yr).
How and when in the star formation sequence do dust grains start to grow into pebbles is a cornerstone question to both star and planet formation. We compute the polarized radiative transfer from a model solar-type protostellar core, using the POLARI
S code, aligning the dust grains with the local magnetic field, following the radiative torques (RATs) theory. We test the dependency of the resulting dust polarized emission with the maximum grain size of the dust size distribution at the envelope scale, from amax = 1 micron to 50 micron. Our work shows that, in the framework of RAT alignment, large dust grains are required to produce polarized dust emission at levels similar to those currently observed in solar-type protostellar envelopes at millimeter wavelengths. Considering the current theoretical dificulties to align a large fraction of small ISM-like grains in the conditions typical of protostellar envelopes, our results suggest that grain growth (typically > 10 micron) might have already significantly progressed at scales 100-1000 au in the youngest objects, observed less than 10^5 years after the onset of collapse. Observations of dust polarized emission might open a new avenue to explore dust pristine properties and describe, for example, the initial conditions for the formation of planetesimals.
We investigate the young (proto)stellar population in NGC 2023 and the L 1630 molecular cloud bordering the HII region IC 434, using Spitzer IRAC and MIPS archive data, JCMT SCUBA imaging and spectroscopy as well as targeted BIMA observations of one
of the Class 0 protostars, NGC 2023 MM1. We have performed photometry of all IRAC and MIPS images, and used color-color diagrams to identify and classify all young stars seen within a 22x26 field along the boundary between IC 434 and L 1630. For some stars, which have sufficient optical, IR, and/or sub-millimeter data we have also used the online SED fitting tool for a large 2D archive of axisymmetric radiative transfer models to perform more detailed modeling of the observed SEDs. We identify 5 sub-millimeter cores in our 850 and 450 micron SCUBA images, two of which have embedded class 0 or I protostars. Observations with BIMA are used to refine the position and characteristics of the Class 0 source NGC 2023 MM 1. These observations show that it is embedded in a very cold cloud core, which is strongly enhanced in NH2D. We find that HD 37903 is the most massive member of a cluster with 20 -- 30 PMS stars. We also find smaller groups of PMS stars formed from the Horsehead nebula and another elephant trunk structure to the north of the Horsehead. We refine the spectral classification of HD 37903 to B2 Ve. Our study shows that the expansion of the IC 434 HII region has triggered star formation in some of the dense elephant trunk structures and compressed gas inside the L 1630 molecular cloud. This pre-shock region is seen as a sub-millimeter ridge in which stars have already formed. The cluster associated with NGC 2023 is very young, and has a large fraction of Class I sources.
Using data from the Extreme-ultraviolet Imaging Spectrometer aboard Hinode, we have studied the coronal plasma in the core of two active regions. Concentrating on the area between opposite polarity moss, we found emission measure distributions having
an approximate power-law form EM$propto T^{2.4}$ from $log,T = 5.5$ up to a peak at $log,T = 6.55$. We show that the observations compare very favorably with a simple model of nanoflare-heated loop strands. They also appear to be consistent with more sophisticated nanoflare models. However, in the absence of additional constraints, steady heating is also a viable explanation.
We present high-angular (~0.4) resolution mid-infrared (MIR) polarimetric observations in the 8.7 ${mu}$m and 11.6 ${mu}$m filters of Cygnus A using CanariCam on the 10.4-m Gran Telescopio CANARIAS. A highly polarized nucleus is observed with a degre
e of polarization of 11${pm}$3% and 12${pm}$3% and position angle of polarization of 27${pm}$8 degrees and 35${pm}$8 degrees in a 0.38 (~380 pc) aperture for each filter. The observed rising of the polarized flux density with increasing wavelength is consistent with synchrotron radiation from the pc-scale jet close to the core of Cygnus A. Based on our polarization model, the synchrotron emission from the pc-scale jet is estimated to be 14% and 17% of the total flux density in the 8.7 ${mu}$m and 11.6 ${mu}$m filters, respectively. A blackbody component with a characteristic temperature of 220 K accounts for >75% of the observed MIR total flux density. The blackbody emission arises from a combination of (1) dust emission in the torus; and (2) diffuse dust emission around the nuclear region, but the contributions of the two components cannot be well constrained in these observations.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
