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Register a new userStar Formation Across the W3 Complex
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Carlos Rom\\'an-Z\\'u\\~niga
Publication date
2015
fields
Physics
and research's language is
English
Authors
C. G. Roman-Zu~niga
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We present a multi-wavelength analysis of the history of star formation in the W3 complex. Using deep, near-infrared ground-based images, combined with images obtained with Spitzer and Chandra observatories, we identified and classified young embedded sources. We identified the principal clusters in the complex, and determined their structure and extension. We constructed extinction-limited samples for five principal clusters, and constructed K-band luminosity functions (KLF) that we compare with those of artificial clusters with varying ages. This analysis provided mean ages and possible age spreads for the clusters. We found that IC 1795, the centermost cluster of the complex, still hosts a large fraction of young sources with circumstellar disks. This indicates that star formation was active in IC 1795 as recently as 2 Myr ago, simultaneous to the star forming activity in the flanking embedded clusters, W3-Main and W3(OH). A comparison with carbon monoxide emission maps indicates strong velocity gradients in the gas clumps hosting W3-Main and W3(OH) and show small receding clumps of gas at IC 1795, suggestive of rapid gas removal (faster than the T Tauri timescale) in the cluster forming regions. We discuss one possible scenario for the progression of cluster formation in the W3 complex. We propose that early processes of gas collapse in the main structure of the complex could have defined the progression of cluster formation across the complex with relatively small age differences from one group to another. However, triggering effects could act as catalysts for enhanced efficiency of formation at a local level, in agreement with previous studies.
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We observed three high-mass star-forming regions in the W3 high-mass star formation complex with the Submillimeter Array and IRAM 30 m telescope. These regions, i.e. W3 SMS1 (W3 IRS5), SMS2 (W3 IRS4) and SMS3, are in different evolutionary stages and are located within the same large-scale environment, which allows us to study rotation and outflows as well as chemical properties in an evolutionary sense. While we find multiple mm continuum sources toward all regions, these three sub-regions exhibit different dynamical and chemical properties, which indicates that they are in different evolutionary stages. Even within each subregion, massive cores of different ages are found, e.g. in SMS2, sub-sources from the most evolved UCHII region to potential starless cores exist within 30 000 AU of each other. Outflows and rotational structures are found in SMS1 and SMS2. Evidence for interactions between the molecular cloud and the HII regions is found in the 13CO channel maps, which may indicate triggered star formation.
The Perseus Arm is the closest Galactic spiral arm from the Sun, offering an excellent opportunity to study in detail its stellar population. However, its distance has been controversial with discrepancies by a factor of two. Kinematic distances are in the range 3.9-4.2 kpc as compared to 1.9-2.3 kpc from spectrophotometric and trigonometric parallaxes, reinforcing previous claims that this arm exhibits peculiar velocities. We used the astrometric information of a sample of 31 OB stars from the star-forming W3 Complex to identify another 37 W3 members and to derive its distance from their Gaia-DR2 parallaxes with improved accuracy. The Gaia-DR2 distance to the W3 Complex,2.14$^{+0.08}_{-0.07}$ kpc, coincides with the previous stellar distances of $sim$ 2 kpc. The Gaia-DR2 parallaxes tentatively show differential distances for different parts of the W3 Complex: W3 Main, located to the NE direction, is at 2.30$^{+0.19}_{-0.16}$ kpc, the W3 Cluster (IC 1795), in the central region of the complex, is at 2.17$^{+0.12}_{-0.11}$ kpc, and W3(OH) is at 2.00$^{+0.29}_{-0.23}$ kpc to the SW direction. The W3 Cluster is the oldest region, indicating that it triggered the formation of the other two star-forming regions located at the edges of an expanding shell around the cluster.
In this work we have carried out an in-depth analysis of the young stellar content in the W3 GMC. The YSO population was identified and classified in the IRAC/MIPS color-magnitude space according to the `Class scheme and compared to other classifications based on intrinsic properties. Class 0/I and II candidates were also compared to low/intermediate-mass pre-main-sequence stars selected through their colors and magnitudes in 2MASS. We find that a reliable color/magnitude selection of low-mass PMS stars in the infrared requires prior knowledge of the protostar population, while intermediate mass objects can be more reliably identified. By means of the MST algorithm and our YSO spatial distribution and age maps we investigated the YSO groups and the star formation history in W3. We find signatures of clustered and distributed star formation in both triggered and quiescent environments. The central/western parts of the GMC are dominated by large scale turbulence likely powered by isolated bursts of star formation that triggered secondary star formation events. Star formation in the eastern high density layer also shows signs of extended periods of star formation. While our findings support triggering as a key factor for inducing and enhancing some of the major star forming activity in the HDL (e.g., W3 Main/W3(OH)), we argue that some degree of quiescent or spontaneous star formation is required to explain the observed YSO population. Our results also support previous studies claiming an spontaneous origin for the isolated massive star(s) powering KR 140.
We investigate the star formation activity in the molecular complex associated with the Galactic HII region Sh2-90, using radio-continuum maps obtained at 1280 MHz and 610 MHz, Herschel Hi-GAL observations at 70 -- 500 microns, and deep near-infrared observation at JHK bands, along with Spitzer observations. Sh2-90 presents a bubble morphology in the mid-IR (size ~ 0.9 pc x 1.6 pc). Radio observations suggest it is an evolved HII region with an electron density ~ 144 cm^-3, emission measure ~ 6.7 x 10^4 cm^-6 pc and a ionized mass ~ 55 Msun. From Hi-GAL observations it is found that the HII region is part of an elongated extended molecular cloud (size ~ 5.6 pc x 9.7 pc, H_2 column density >= 3 x 10^21 cm^-2 and dust temperature 18 -- 27 K) of total mass >= 1 x 10^4 Msun. We identify the ionizing cluster of Sh2-90, the main exciting star being an O8--O9 V star. Five cold dust clumps (mass ~ 8 -- 95 Msun), four mid-IR blobs around B stars, and a compact HII region are found at the edge of the bubble.The velocity information derived from CO (J=3-2) data cubes suggests that most of them are associated with the Sh2-90 region. 129 YSOs are identified (Class I, Class II, and near-IR excess sources). The majority of the YSOs are low mass (<= 3 Msun) sources and they are distributed mostly in the regions of high column density. Four candidate Class 0/I MYSOs have been found; they will possibly evolve to stars of mass >= 15 Msun. We suggest multi-generation star formation is present in the complex. From the evidences of interaction, the time scales involved and the evolutionary status of stellar/protostellar sources, we argue that the star formation at the immediate border/edges of Sh2-90 might have been triggered by the expanding HII region. However, several young sources in this complex are probably formed by some other processes.
The giant HII region W31 hosts the populous star cluster W31-CL and others projected on or in the surroundings. The most intriguing object is the stellar cluster SGR1806-20, which appears to be related to a Luminous Blue Variable (LBV) - a luminous supergiant star. We used the deep VVV J-,H-and K$_s$-bands photometry combined with 2MASS data in order to address the distance andother physical and structural properties of the clusters W31-CL, BDS 113 and SGR1806-20. Field-decontaminated photometry was used to analyse colour-magnitude diagrams and stellar radial density profiles, using procedures that our group has developed and employed in previous studies. We concludethat the clusters W31-CL and BDS113 are located at 4.5kpc and 4.8kpc and have ages of 0.5Myr and 1Myr, respectively. This result, together with the pre-main sequence (PMS) distribution in the colour-magnitude diagram, characterises them as members of the W31 complex. The present photometry detects the stellar content, addressed in previous spectroscopic classifications, in the direction of thecluster SGR1806-20, including the LBV, WRs, and foreground stars. We derive an age of 10$pm$4Myr and a distance of d=8.0$pm$1.95kpc. The cluster is extremely absorbed, with AV= 25mag. Thepresent results indicate that SGR1806-20 is more distant by a factor 1.8 with respect to the W31 complex, and thus not physically related to it.
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