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تسجيل مستخدم جديدThe physical and chemical structure of Sagittarius B2. II. Continuum millimeter emission of SgrB2(M) and SgrB2(N) with ALMA
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The high-mass star forming sites SgrB2(M) and SgrB2(N) have been the target of numerous studies, revealing e.g. a rich chemistry. We want to characterize their physical and chemical structure using ALMA high-angular resolution observations at mm wavelengths, reaching spatial scales of about 4000 au, and covering the whole band 6 (from 211 to 275 GHz). In order to determine the continuum emission in line-rich sources, we use a new statistical method: STATCONT. We detect 27 continuum sources in SgrB2(M) and 20 in SgrB2(N). We study the continuum emission across the ALMA band 6, and compare it with previous SMA 345 GHz and VLA 40 GHz observations, to study the nature of the sources detected. The brightest sources are dominated by (partially optically thick) dust emission, while there is an important degree of contamination from ionized gas free-free emission in weaker sources. While the total mass in SgrB2(M) is distributed in many fragments, most of the mass in SgrB2(N) arises from a single object, with filamentary-like structures converging towards the center. There seems to be a lack of low-mass dense cores in both regions. We determine H2 volume densities for the cores of about 10^5-10^7 Msun pc^-3, one to two orders of magnitude higher than the stellar densities of super star clusters. In general, SgrB2(N) is chemically richer than SgrB2(M). There seems to be a correlation between the chemical richness and the mass of the fragments, with more massive clumps being more chemically rich. Both SgrB2(N) and SgrB2(M) harbour a cluster of hot molecular cores. We compare the continuum images with predictions from a detailed 3D radiative transfer model that reproduces the structure of SgrB2 from 45 pc down to 100 au. This dataset, together with ongoing projects in the range 5 to 200 GHz, better constrain the 3D structure of SgrB2, and allow us to understand its physical and chemical structure.
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The giant molecular cloud Sagittarius B2 (hereafter SgrB2) is the most massive region with ongoing high-mass star formation in the Galaxy. In the southern region of the 40-pc large envelope of SgrB2, we encounter the SgrB2(DS) region which hosts more
than 60 high-mass protostellar cores distributed in an arc shape around an extended HII region. We use the Very Large Array in its CnB and D configurations, and in the frequency bands C (4--8 GHz) and X (8--12 GHz) to observe the whole SgrB2 complex. Continuum and radio recombination line maps are obtained. We detect radio continuum emission in SgrB2(DS) in a bubble-shaped structure. From 4 to 12 GHz, we derive a spectral index between -1.2 and -0.4, indicating the presence of non-thermal emission. We decompose the contribution from thermal and non-thermal emission, and find that the thermal component is clumpy and more concentrated, while the non-thermal component is more extended and diffuse. The radio recombination lines in the region are found to be not in local thermodynamic equilibrium (LTE) but stimulated by the non-thermal emission. The thermal free-free emission is likely tracing an HII region ionized by an O7 star, while the non-thermal emission can be generated by relativistic electrons created through first-order Fermi acceleration. We have developed a simple model of the SgrB2(DS) region and found that first-order Fermi acceleration can reproduce the observed flux density and spectral index.
We have used an unbiased, spectral line-survey that covers the frequency range from 211 to 275 GHz and was obtained with ALMA (angular resolution of 0.4 arcsec) to study the small-scale structure of the dense gas in Sagittarius B2 (north). Eight fila
ments are found converging to the central hub and extending for about 0.1 pc. The spatial structure, together with the presence of the massive central region, suggest that these filaments may be associated with accretion processes. In order to derive the kinematic properties of the gas in a chemically line-rich source like Sgr B2(N), we have developed a new tool that stacks all the detected transition lines of any molecular species. This permits to increase the signal-to-noise ratio of our observations and average out line blending effects, which are a common problem in line-rich regions. We derive velocity gradients along the filaments of about 20-100 km s$^{-1}$ pc$^{-1}$, which are 10-100 times larger than those typically found on larger scales (1 pc) in other star-forming regions. The mass accretion rates of individual filaments are about 0.05 M$_odot$ yr$^{-1}$, which result in a total accretion rate of 0.16 M$_odot$ yr$^{-1}$. Some filaments harbor dense cores that are likely forming stars and stellar clusters. The stellar content of these dense cores is on the order of 50% of the total mass. We conclude that the cores may merge in the center when already forming stellar clusters but still containing a significant amount of gas, resulting in a damp merger. The high density and mass of the central region, combined with the presence of converging filaments with high mass, high accretion rates and embedded dense cores already forming stars, suggest that Sgr B2(N) may have the potential to evolve into a super stellar cluster.
We model the dust and free-free continuum emission in the high-mass star-forming region Sagittarius B2 in order to reconstruct the three-dimensional density and dust temperature distribution, as a crucial input to follow-up studies of the gas velocit
y field and molecular abundances. We employ the three-dimensional radiative transfer program RADMC-3D to calculate the dust temperature self-consistently, provided a given initial density distribution. This density distribution of the entire cloud complex is then recursively reconstructed based on available continuum maps, including both single-dish and high-resolution interferometric maps covering a wide frequency range (40 GHz - 4 THz). The model covers spatial scales from 45 pc down to 100 au, i.e. a spatial dynamic range of 10^5. We find that the density distribution of Sagittarius B2 can be reasonably well fitted by applying a superposition of spherical cores with Plummer-like density profiles. In order to reproduce the spectral energy distribution, we position Sgr B2(N) along the line of sight behind the plane containing Sgr B2(M). We find that the entire cloud complex comprises a total gas mass of 8.0 x 10^6 Msun within a diameter of 45 pc, corresponding to an averaged gas density of 170 Msun/pc^3. We estimate stellar masses of 2400 Msun and 20700 Msun and luminosities of 1.8 x 10^6 Lsun and 1.2 x 10^7 Lsun for Sgr B2(N) and Sgr B2(M), respectively. We report H_2 column densities of 2.9 x 10^24 cm^-2 for Sgr B2(N) and 2.5 x 10^24 cm^-2 for Sgr B2(M) in a 40 beam. For Sgr B2(S), we derive a stellar mass of 1100 Msun, a luminosity of 6.6 x 10^5 Lsun and a H_2 column density of 2.2 x 10^24 cm^-2 in a 40 beam. We calculate a star formation efficiency of 5% for Sgr B2(N) and 50% for Sgr B2(M), indicating that most of the gas content in Sgr B2(M) has already been converted to stars or dispersed.
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