J/ApJ/841/109 Cloud decomposition & SFR measurements (Ochsendorf+, 2017)
What sets the massive star formation rates and efficiencies of giant molecular clouds? Ochsendorf B.B., Meixner M., Roman-Duval J., Rahman M., Evans N.J. <Astrophys. J., 841, 109 (2017)> =2017ApJ...841..109O
ADC_Keywords: Magellanic Clouds ; Molecular clouds ; H II regions ; YSOs ; Carbon monoxide ; Infrared sources Keywords: H II regions ; ISM: clouds ; Magellanic Clouds ; stars: formation ; stars: massive Abstract: Galactic star formation scaling relations show increased scatter from kpc to sub-kpc scales. Investigating this scatter may hold important clues to how the star formation process evolves in time and space. Here, we combine different molecular gas tracers, different star formation indicators probing distinct populations of massive stars, and knowledge of the evolutionary state of each star-forming region to derive the star formation properties of ∼150 star-forming complexes over the face of the Large Magellanic Cloud (LMC). We find that the rate of massive star formation ramps up when stellar clusters emerge and boost the formation of subsequent generations of massive stars. In addition, we reveal that the star formation efficiency of individual giant molecular clouds (GMCs) declines with increasing cloud gas mass (Mcloud). This trend persists in Galactic star-forming regions and implies higher molecular gas depletion times for larger GMCs. We compare the star formation efficiency per freefall time (εff) with predictions from various widely used analytical star formation models. While these models can produce large dispersions in εff similar to those in observations, the origin of the model-predicted scatter is inconsistent with observations. Moreover, all models fail to reproduce the observed decline of εff with increasing Mcloud in the LMC and the Milky Way. We conclude that analytical star formation models idealizing global turbulence levels and cloud densities and assuming a stationary star formation rate (SFR) are inconsistent with observations from modern data sets tracing massive star formation on individual cloud scales. Instead, we reiterate the importance of local stellar feedback in shaping the properties of GMCs and setting their massive SFR. Description: We use the Magellanic Mopra Assesment (MAGMA) DR3 (Wong+, J/ApJS/197/16 2017 in prep.) CO intensity map to determine molecular masses using Mmol=αCOLCO, where LCO is the CO luminosity and αCO=8.6(K.km/s.pc2)-1 is the proportionality constant appropriate for the LMC (Bolatto+ 2013ARA&A..51..207B). In addition, we use the dust-based molecular hydrogen map of Jameson+ (2016ApJ...825...12J ; hereafter J16). The J16 map combines far-infrared dust emission (modeled with a single-temperature blackbody modified by a broken power-law emissivity; Gordon+ 2014ApJ...797...85G) and atomic hydrogen maps to estimate the H2 distribution. See section 2.1 for further explanations. We utilize Ochsendorf+ (2016ApJ...832...43O) MYSO catalog to obtain a census of massive star formation by counting the number of MYSOs in each cloud identified in the dendrogram decomposition. See section 2.2 for further explanations. We correct the Hα emission (from the Southern H-Alpha Sky Survey Atlas (SHASSA); Gaustad+ 2001PASP..113.1326G) for extinction using 24um emission (from Spitzer's Surveying the Agents of a Galaxy's Evolution; Meixner+ 2013, J/AJ/146/62). See section 2.3 for further explanations. File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file table4.dat 92 220 *MAGMA cloud decomposition with MYSO SFR parameters table5.dat 92 52 *MAGMA cloud decomposition with Ha+24um SFR parameters table6.dat 92 99 *J16 cloud decomposition with MYSO SFR parameters table7.dat 92 58 *J16 cloud decomposition with Ha+24um SFR parameters
Note on table*.dat: There are a variety of ways to separate emission into clouds and measure their masses. We employ two methods: one uses the traditional conversion from CO luminosity to mass (denoted MAGMA; Magellanic Mopra Assesment, Wong+, J/ApJS/197/16 ; 2017 in prep.), and the second uses the dust continuum emission (denoted J16; Jameson+ 2016ApJ...825...12J). Similarly, we use two methods to measure the SFR: one counts massive young stellar objects (MYSOs) and uses an initial mass function and a characteristic age, as employed in studies of nearby Galactic clouds (denoted MYSO); the other uses the diffuse emission from gas and dust affected by star formation, as is traditional in extragalactic studies (denoted Ha+24um).
See also: J/ApJ/686/948 : CO in extragalactic giant molecular clouds (Bolatto+, 2008) J/ApJS/178/56 : CO observations of LMC Giant Molecular clouds (Fukui+, 2008) J/AJ/136/18 : LMC SAGE. New candidate YSOs (Whitney+, 2008) J/ApJS/181/321 : Properties of Spitzer c2d dark clouds (Evans+, 2009) J/ApJS/184/172 : High- and intermediate-mass YSOs in the LMC (Gruendl+, 2009) J/ApJ/699/1092 : Giant molecular clouds (SRBY) (Heyer+, 2009) J/ApJS/184/1 : Molecular clouds in the LMC by NANTEN. II. (Kawamura+, 2009) J/ApJ/707/1417 : HST view of YSOs in the LMC (Vaidya+, 2009) J/ApJ/723/1019 : Galactic SFR and gas surface densities (Heiderman+, 2010) J/ApJ/709/424 : HII regions identified with WMAPS and GLIMPSE (Murray+, 2010) J/ApJ/723/492 : Physical data of GRS molecular clouds (Roman-Duval+, 2010) J/ApJS/197/16 : CO observations of LMC molecular clouds (MAGMA). (Wong+, 2011) J/ApJ/755/40 : HII regions in the MCs from MCELS (Pellegrini+, 2012) J/AJ/146/62 : HERschel HERITAGE in Magellanic Clouds (Meixner+, 2013) J/ApJ/789/81 : Multiwavelength survey of HII regions in NGC300 (Faesi+, 2014) J/AJ/148/124 : Herschel key program Heritage (Seale+, 2014) J/ApJS/220/11 : SEDs of Spitzer YSOs in the Gould Belt (Dunham+, 2015) J/ApJ/806/231 : MISFITS survey: HCO+ obs. of Spitzer YSOs (Heiderman+, 2015) J/A+A/588/A29 : Star formation in massive clumps in Milky Way (Heyer+, 2016) J/ApJ/833/229 : Star forming cloud-GMC complexes (Lee+, 2016) J/ApJ/831/73 : Galactic MCs with HII regions (Vutisalchavakul+, 2016) Byte-by-byte Description of file: table.dat
Bytes Format Units Label Explanations
1- 3 I3 --- Seq [1/220] Running sequence number (1) 4 A1 --- --- [_] 5- 7 A3 --- ClD Cloud decomposition method (1) 8 A1 --- --- [_] 9- 12 A4 --- SFt SF tracer (1) 14- 19 F6.3 deg RAdeg [71.8/89] Right Ascension (J2000) 21- 27 F7.3 deg DEdeg [-72/-65] Declination (J2000) 29 I1 --- Type [1/3] Kawamura+ 2009, J/ApJS/184/1, giant molecular cloud (GMC) type (2) 31- 38 E8.2 Msun Mass [2440/3.6e+06] Total mass 40- 47 E8.2 pc Rad [6.8/171] Total radius 49- 56 E8.2 Msun/Myr SFR [0/91200] Star formation rate (3) 58- 65 E8.2 Myr-1 SFE [0/0.5] Star formation 'efficiency'; SFR/Mcloud 67- 74 E8.2 --- SFEff [0/4.7] Star formation efficiency per free fall time; (SFR/Mcloud)*tff 76- 83 E8.2 Myr tff [5.9/19.7] Free-fall time 85- 92 E8.2 km/s sigV [0.4/5]? CO velocity dispersion; intensity weighted (only for tables 4 and 5)
Note (1): Cloud unique IDs are composed of Seq, ClD and SFt with ClD, the cloud decomposition being "mag" for MAGMA or "j16" and the SF tracer, "myso" or "ha24". See the Note on the table. Note (2): Type as follows: 1 = Type I: no signature of massive star formation; 2 = Type II: associated with relatively small HII region(s); 3 = Type III: associated with both HII region(s) and young stellar cluster(s). Note (3): From MYSOs for Tables 4 and 6 or from Hα+24um for Tables 5 & 7.
History: From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 22-Jan-2018
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