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 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 2013ARA&A..51..207B). In addition,
we use the dust-based molecular hydrogen map of Jameson+
(2016ApJ...825...12J 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 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 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 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 83 99 *J16 cloud decomposition with MYSO SFR parameters
table7.dat 83 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 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[4567].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