Presentation on theme: "Progresses on our understanding the processes of star formation in the Milky Way from Herschel observations Davide Elia INAF-IAPS, Roma Part II The Herschel."— Presentation transcript:
Progresses on our understanding the processes of star formation in the Milky Way from Herschel observations Davide Elia INAF-IAPS, Roma Part II The Herschel photometric surveys
Herschel and star formation The wavelength range covered by the cameras on board Herschel contains the emission peak of the cold dust. It is suited for studying the dense clouds and the early stages of star formation!
Nature of the compact sources Warm Cores SED sources are under-luminous with respect to UCHII/HotCores of similar envelope mass Concurring indications suggesting that the dominant source in the Warm Core objects is not yet on the ZAMS ZAMS ACCRETION Molinari et al Hot Core Warm Core In a pre-Herschel SED analysis of sample of 42 intermediate and high- mass star forming region from the sample of Molinari et al. (1996), a Class 0-I-II sequence analogous to the low-mass regime was suggested: Warm Cores to Hot Cores to HII-driving objects
Nature of the compact sources ZAMS ACCRETION Krumholz & McKee (2008) ≈1 g cm -2 A significant fraction of the clumps should be already forming high-mass protostars (M≥10M ) Molinari Elia Problem: Sources in Hi-GAL are mostly clumps, while SED models are available for single YSOs (Robitaille et al. 2006)
Herschel photometric surveys of star forming regions Gould Belt HOBYS Hi-GAL André et al. 2010, A&A, 518, L102 Motte et al. 2010, A&A, 518, L hrs Molinari et al. 2010, PASP, 122, hrs 900 hrs
Photometric imaging of nearby (d < 0.5 kpc) molecular clouds Formation of solar-type stars Reasonably well established evolutionary sequence, but physics of early stages unclear What determines the distribution of stellar masses = the IMF? What generates prestellar cores & what governs their evolution to protostars? Timescale of core/star formation? Quasi-static or dynamic process? ?
Aquila rift and Polaris flare André et al. 2010, A&A, 518, L102; Könives et al. 2010, A&A, 518, L106
BTW: How to calculate N(H 2 ) and T maps? Pixel-to-pixel grey-body fit Regrid the Herschel maps onto the map at the largest wavelength available (usually λ MAX = 500 μm). Reconvolve them with the Herschel FWHM at λ MAX Perform the pixel-to-pixel fit (time consuming: parallel computing is recommended)
Aquila rift and Polaris flare André et al. 2010, A&A, 518, L102; Könives et al. 2010, A&A, 518, L106 Prestellar cores are only observed above the threshold A V = 7 because they form out of a filamentary background and only the supercritical, gravitationally unstable filaments are able to fragment into bound cores.
Two First Hydrostatic Cores in Perseus Pezzuto et al. 2012, A&A, 547, A54 FHC Difficult to see it, because it is: Short-lived (t = yr) Invisible in the MIR Hard to resolve, even at near distances (size = several AU) Perseus, d 235 pc
The two sources are situated a few 10^3 AU apart, corresponding to a few Jeans lengths. It is then possible that these two sources formed at almost the same time from the fragmentation of a larger structure. Two First Hydrostatic Cores in Perseus Pezzuto et al. 2012, A&A, 547, A54 envelope: T = 9 K, M = 7.3 M ʘ envelope: T = 9.4 K, M = 8 M ʘ
Photometric imaging of all the high-mass star forming regions at d < 3 kpc Molecular complexes Distance (kpc) Area (deg 2 ) Vela Mon OB1/NGC2264 Mon R Rosette Cygnus X M 16/M17/Sh NGC 6334/NGC W3/KR NGC W Sh 1044 RCW 794 RCW RCW These data can allow us to determine the importance of external triggers for high-mass star formation in the nearest massive molecular cloud complexes.
O-stars from NGC 2244 Filaments in the Rosette molecular cloud “Confidence map” highligthing the filament junctions Existing infrared clusters and the most massive dense cores (potential sites of future massive star formation) identified in the same data set are overlaid on the image. All sources lie in the proximity of junctions Schneider et al. 2012, A&A 540, L11
O-stars from NGC 2244 Schneider et al. 2012, A&A 540, L11 PDFs of the Rosette molecular cloud
The Vela–C cloud b = 0º It is the cloud “C” of the Vela Molecular Ridge (Murphy & May, 1991) distance = 700 ± 200 pc (Liseau et al. 1992) Site of star formation on a wide range of masses (Massi et al. 2003; Baba et al. 2006) BLAST 250 μm 3 deg 2 HOBYS Giannini et al. 2012, A&A 539, A156
Vela–C - Compact source extraction Sources are searched separately on each map CuTEx: sources detected as local maxima in the curvature map (2nd derivative) An elliptical Gaussian is fitted on them, and geometric parameters estimated A list of sources with S/N>5 is obtained at each λ
Vela–C - SED fitting The SEDs eligible for the grey-body fit have been selected applying few constraints: i) fluxes at least 3 adjacent bands between 160 and 500 μm; ii) without concavities; iii) no peak at 500 μm; iv) spatially resolved at 160 μm; v) not presenting multiple associations at λ ≥ 160 μm; vi) not belonging to the RCW34 region 268 objects selected for fit Giannini et al. 2012, A&A 539, A156
Vela–C - Pre-stellar sources 206 out of 218 starless sources shave been recognized as pre- stellar (~94%, probably affected by selection). In the mass vs size plot, all the unbound starless sources lie below the Bonnor-Ebert mass curve at T = 8 K. To determine if a starless source is gravitationally bound (then pre-stellar), a comparison of its mass with the corresponding Bonnor-Ebert mass has been performed: Giannini et al. 2012, A&A 539, A156
Vela–C - An evolutionary framework Class 0 Although not completely separated, the pre- and proto-stellar core samples show a global trend to populate different regions of the diagram. For proto-stellar cores, L bol is probably underestimated, resulting in an underestimate of their actual age. Giannini et al. 2012, A&A 539, A156
Vela–C - The Source Mass Distribution Vela-C : γ=1.1±0.2 Aquila Rift Könives et al. (2010): γ=1.45±0.2 (M > 0.3 M ʘ ) Orion A Polychroni et al. γ=1.5±0.5, Ikeda et al. γ=1.3±0.1 (M > 9.3 M ʘ ) Orion B Johnstone et al γ=1.5±0.42 Perseus+Serpens+Ophiuchus Enoch et al. 2008: γ=1.3±0.2 (M > 0.8 M ʘ ) Kroupa 2001 Kramer et al This work Chabrier 2005 D< 0.08 pc Giannini et al. 2012, A&A 539, A156