The Lifetimes of Phases in High-Mass Star-Forming Regions

Cara Battersby1 & John Bally2

ABSTRACT

High-mass stars are born in dense molecular clumps, but the nature of this

process is a subject of much debate. Whether the clusters which form high-mass

stars are born through slow, hydrostatic, global contraction of dense molecular

clumps or quickly and dynamically, is a key question. We investigate the lifetimes

of phases of high-mass star formation in dense molecular clumps by comparing

the relative fraction in each phase in a 2×2 degree region centered at [`, b] =

[30◦, 0◦]. Of all the regions capable of forming high-mass stars on ∼ 1 pc scales,

based on a surface density threshold, the starless phase lasts approximately 0.6

- 1.7 Myr (70%) and the star-forming phase lasts approximately 0.3 - 0.7 Myr

(30%). The starless/star-forming lifetime is the estimated time during which an

entire ∼ 1 pc region exceeds a critical surface density and appears devoid/full

of massive stars (as traced by heated dust or UV-excited 8 micron emission).

Regions need to have accumulated enough material to exceed the critical sur-

face density to be considered in this phase and also can not have significantly

cleared the environment through feedback. The relative lifetimes of the star-

less vs. star-forming phases are robustly determined, while the absolute lifetime

determinations from the maser and UCHII region lifetimes are less precise and

will benefit from larger-scale, higher resolution analyses. The 6.7 GHz Class II

CH3OH masers tend to be found at the intersection of starless and star-forming

High Surface Density (HiSD) regions, indicating that they exist for a short time

when a high-mass star turns on.

Subject headings: ISM: kinematics and dynamics — dust, extinction — HII re-

gions — radio emission lines — stars: formation

1. Introduction

High-mass stars dominate the life cycle of gas and dust in our Galaxy, driving the

dynamics in the interstellar medium and lighting up the universe around us. The transition

1Harvard-Smithsonian Center for Astrophysics

2Center for Astrophysics and Space Astronomy – University of Colorado Boulder

– 2 –

of Giant Molecular Clouds (GMCs) into high-mass stars and stellar clusters is fundamental

to our understanding of the global process of star formation, the origin of the Initial Mass

Function (IMF), planet formation in star clusters, and star formation rates in nearby galaxies.

We have yet to fully understand the formation, early evolution, and lifetimes of high-mass

star-forming regions and this quest remains a cornerstone of modern astrophysics. Whether

star clusters and high-mass stars form as the result of slow, equilibrium global collapse of

clumps (e.g., Tan et al. 2006) over several free fall times or if they collapse quickly on the

order of a free-fall time (e.g., Elmegreen 2000, 2007; Hartmann & Burkert 2007), perhaps

through large scale accretion along filaments (Myers 2009), remains an open question.

To study the formation, early evolution, and lifetimes of high-mass star-forming regions,

we investigate the earliest phases of high-mass star formation. Young, embedded clusters

form from high-density (n(H2 ∼ 104 - 107 cm−3) clumps , ∼ 0.5 - 1 pc radii, temperatures

of ∼50-200 K, and masses & 102 to 103 M� (Lada & Lada 2003). The precursors to these

clusters to are expected to be colder than the clumps actively forming stars. Hence, searches

for proto-cluster-forming regions typically target the longer wavelengths where the dust

continuum peaks in cold, dense clumps (e.g., BGPS, ATLASGAL, Hi-GAL, Aguirre et al.

2011; Schuller et al. 2009; Molinari et al. 2010). These clumps can also be seen in silhouette

as Infrared Dark Clouds (IRDCs, Egan et al. 1998; Perault et al. 1996; Omont et al. 2003)

absorbing the diffuse background Galactic mid-IR light. Due to their cold temperatures (T

< 20 K, Pillai et al. 2006) and high densities (> 105 cm−3) the highest-mass IRDCs (M >

103 M�) are commonly cited as the birthplaces for high-mass stars and stellar clusters (e.g.,

Rathborne et al. 2006; Beuther et al. 2007; Parsons et al. 2009; Battersby et al. 2010), yet not

all IRDCs will form high-mass stars (Kauffmann & Pillai 2010). Additionally, the selection

of IRDCs is inherently biased becasue their identification requires being on the near-side of

a bright-mid-IR background.

Battersby et al. (2011) showed that the physical properties of dust continuum clumps

(temperature and column density) can be used to distinguish the pre-star-forming and ac-

tively star-forming populations, independent of whether or not they are dark in the mid-IR.

Unlike other similar analyses (e.g., Chambers et al. 2009; Miettinen 2012; Wilcock et al.

2012; Tackenberg et al. 2012; Parsons et al. 2009, discussed in §3.5), all pixels above a

surface density threshold are analyzed and binned into either ‘actively star-forming’ or ‘qui-

escent’ categories. Instead of IRDCs, this analysis targets all pixels above the threshold

column density determined from the sub-mm dust continuum flux and temperature derived

from the Herschel Hi-GAL data in the HiSD regions.

– 3 –

2. Methods

The Galactic plane centered at Galactic longitude 30◦ contains one of the largest con-

centrations of dense gas and dust in the Milky Way. Located near the end of the Galactic

bar and the start of the Scutum-Centaurus spiral arm, this region contains the massive W43

star forming complex at a distance of 5.5 kpc (Zhang et al. 2014) and hundreds of massive

clumps and molecular clouds with more than 80% of the emission having VLSR = 80 to 120

km s−1 implying a kinematic distance between 5 and 9 kpc (Carlhoff et al. 2013).

Data from the Herschel Infrared Galactic Plane Survey (Molinari et al. 2010, Hi-GAL),

the Spitzer legacy project GLIMPSE (Benjamin et al. 2003), and surveys for 6.7 GHz CH3OH

masers (Pestalozzi et al. 2005) were used to investigate a 2◦× 2◦ field centered at [l,b] = [30◦,

0◦]. All data were convolved to a common resolution of 25′′. Dust temperatures and column

densities were determined from modified blackbody fits to background-subtracted Hi-GAL

data from 160 to 500 µmusing methods described in Battersby et al. (2011). The itera-

tive background subtraction removed diffuse Galactic cirrus component whose contribution

(N(H2) < 1022 cm−2) is not included in the discussion of the dense region below Battersby

et al. (2011).

High-mass stars tend to form in regions with surface densities∼ Σ∼ 1 g cm−2 (Krumholz

& McKee 2008), corresponds to a column density N(H2) ∼ 2.1×1023 cm−2. At distances of

several kpc or more, most cores are highly beam-diluted in a 25′′ beam. To derive a realistic

high-mass star-forming column density threshold for cores beam-diluted by a 25′′ beam,

consider a spherical core with a constant column density Σ = 1 g cm−2 for r < rf and a

power-law drop off for r > rfn(r) = nf (r/rf )−p (1)

where p is the density power-law exponent. The Mueller et al. (2002) study of 51 high-mass

star-forming cores found a best-fit central core radius, rf ≈ 1000 AU, and density power-law

index, p = 1.8. This model implies an H2 central density of nf = 6.2 × 107 cm−3, which

integrated over rf = 1000 AU, corresponds to the theoretical surface density threshold for

forming high-mass stars of Σ = 1 g cm−2 and ∼0.35 M� contained within rf .

Integration of this model core along the line of sight and convolution with a 25′′ beam

results in a beam-diluted column density threshold appropriate for the analysis of the Hi-

GAL based column density maps. At typical distances of 5 and 9 kpc toward the ` = 30◦ field

(Ellsworth-Bowers et al., in prep.) a 25′′ beam subtends 0.7 and 1.1 pc and the beam-diluted

column densities of the model cores would be N(H2) = 0.8 and 0.4 × 1022 cm−2, respectively.

Pixels above this column density threshold are referred to as ‘High-Surface Density Regions’

(HiSD regions). The mass enclosed in the beam area ranges from 63 to 83 M�, sufficient to

– 4 –

form a 20 M� star with ∼30% efficiency.

The identification of starless vs. star-forming HiSD regions is based on the approach

from Battersby et al. (2011) and uses the HiSD region temperature and emission at 8 µm to

determine whether it is starless or star-forming. Pixels above the column density threshold

for forming high-mass stars (HiSD regions) are identified. Next, each HiSD region is tagged

as starless or star-forming based on the absence or presence of 8 µm emission. From the

relative fraction of HiSD regions capable of forming high-mass stars in the starless vs. star-

forming phase, we determine the relative lifetimes of each phase. Above our column density

threshold, we also search for 6.7 GHz Class II CH3OH masers from the unbiased searches by

Szymczak et al. (2002) and Ellingsen (1996), as described in §2.2. Using the relative fraction

of HiSD regions containing 6.7 GHz masers and their absolute lifetime of 35,000 years from

the literature (van der Walt 2005), we determine a range of absolute lifetimes for each phase.

Figure 1 demonstrates that the relative lifetimes are insensitive to variations in the

threshold column density from ∼0.3 to 1.3 × 1022 cm−2 (corresponding to distances of 11

and 3 kpc for the model core). Thus, two cutoffs are used to determine relative lifetimes:

N(H2) = 0.4 × 1022 cm−2) dubbed the ‘generous’ and N(H2) = 0.8 × 1022 cm−2) dubbed

the ‘conservative’ cutoff.

2.1. Starless vs. Star-Forming

Battersby et al. (2011) found that Herschel dust continuum sources can be separated

out into starless and star-forming based on the absence pr presence of 24 µm sources, 8 µm

PAH, or excess 4.5 µm emission in Spitzer data (Cyganowski et al. 2008, 2011b,a), masers,

or HII regions. Dust temperature were found to increase with star formation activity. In

this Letter, dust temperature distributions are combined with each HiSD region’s mid-IR

star-formation signatures to determine weather the region is ‘starless’ or ‘star-forming’. This

study is only sensitive to the star-forming signatures of high-mass stars. Therefore, the term

‘starless’ refers only to the absence of high-mass stars forming; the region may support active

low-mass star formation.

Pixels above the column density threshold described above are classified as either starless

of star-forming, based on its temperature and signature at 8 µm. A HiSD region can be mid-

IR-dark (an IRDC absorbing the background 8 µm light), mid-IR-neutral (no signature at 8

µm), or mid-IR-bright (from UV excited PAH emission at 8 µm). Above the column density

thresholds, mid-IR-dark HiSD regions are starless, while the mid-IR-bright HiSD regions are

associated with high-mass star formation. Above the ‘generous’ and ‘conservative’ thresholds

– 5 –

0.0 0.5 1.0 1.5 2.0Column Density x 1022 [cm−2]

0

20

40

60

80

100

Per

cent

30%

70%

0

5.0•103

1.0•104

1.5•104

2.0•104

2.5•104

Num

ber

of

Pix

els

Starless GenerousStarless Conservative

Starry GenerousStarry Conservative

Maser

Fig. 1.— The relative fraction of pixels which are starless (70%) vs. star-forming (30%) is

mostly insensitive to column density cutoff over a reasonable range of thresholds (N(H2) ∼0.3 - 1.3 × 1022 cm−2) in a 25′′ beam. The left y-axis and dashed lines show the percentage

of pixels which are starless (cyan and green using the ‘generous’ and ‘conservative’ identi-

fication methods respectively) vs. star-forming (magenta and orange using the ‘generous’

and ‘conservative’ identification methods respectively) or which contain a maser (red) as a

function of the column density threshold selected. The right y-axis and solid lines show the

column density distribution of the same populations.

– 6 –

(N(H2) = 0.4 and 0.8 × 1022 cm−2, respectively), a Gaussian fit to the mid-IR-dark and

mid-IR-bright temperature distributions are used tto classify all HiSD regions as starless or

star-forming.

As with the column densities, two different cutoffs based on the temperature distri-

butions are used, a ‘generous’ and ‘conservative’ cutoff corresponding to 3 and 2 σ cuts,

respectively. A HiSD region is classified as starless if it: 1) is mid-IR-dark and falls within 2

(or 3 for the ‘generous’ cutoff) σ of the mid-IR-dark temperature distribution (Conservative:

T = 11-28 K, Generous: T = 9-32 K) or 2) is mid-IR-neutral and falls within the same

distribution, with an upper limit at 25 K so as not to overlap with the star-forming popu-

lation. Conversely, a HiSD region is classified as star-forming if it: 1) is mid-IR-bright and

falls within 2 (or 3 for the ‘generous’ cutoff) σ of the mid-IR-bright temperature distribution

(Conservative: 21-42 K, Generous: 12-47 K) or 2) is mid-IR-neutral and falls within the same

distribution, with a lower limit at 25 K so as not to overlap with the starless population.

2.2. Associated Methanol Maser

While H2O, OH, and SiO masers are associated with young and post main-sequence

stars, methanol masers tend to be only found in regions of massive star formation. Thus,

6.7 GHz Class II CH3OH masers were selected from the unbiased Galactic plane searches

by Szymczak et al. (2002) and Ellingsen (1996) as compiled by Pestalozzi et al. (2005).

The ‘sizes’ of the methanol masers are defined by a circle with a radius determined by the

positional accuracy of the observations (30′′ and 36′′ respectively for Szymczak et al. 2002;

Ellingsen 1996). While methanol maser emission comes from very small areas of the sky

(e.g., Walsh et al. 1998; Minier et al. 2001), they are often clustered. Szymczak et al. (2002);

Ellingsen (1996) find multiple maser spots towards the majority of sources, so these methanol

maser “sizes” are meant to represent the extent of the star-forming region. Future higher

sensitivity and resolution unbiased searches (e.g., the 6 GHz multi-beam maser survey, Green

et al. 2009) will certainly improve this characterization.

3. Lifetimes

3.1. Discussion of Uncertainties

Several assumptions were made in determining the relative lifetimes of the starless and

star-forming stages for high-mass star-forming regions: i) The sample is complete and un-

biased in time and space. ii) Each HiSD region represents a region which is forming or

– 7 –

30.300 30.200 30.100 30.000 29.900 29.800

0.1

00

0.0

00

-0.1

00

-0.2

00

-0.3

00

-0.4

00

Galactic longitude

Gala

ctic

lati

tud

e

Temperature8 micron emission

Column Density

30.300 30.200 30.100 30.000 29.900 29.800

0.1

00

0.0

00

-0.1

00

-0.2

00

-0.3

00

-0.4

00

Galactic longitude

Gala

ctic

lati

tud

e

Generous

Starless

Starry

Methanol Maser

30.300 30.200 30.100 30.000 29.900 29.800

0.1

00

0.0

00

-0.1

00

-0.2

00

-0.3

00

-0.4

00

Galactic longitude

Gala

ctic

lati

tud

e

Conservative

Starless

Starry

Methanol Maser

Fig. 2.— Depiction of starless (white contour), star-forming (black contour), and maser

(green contour) associated HiSD regions plotted on a three-color image in which red is the

temperature, blue is the column density, and green is the 8 µm emission. Predominantly blue

regions in this map are starless (cold, high column density, and 8 µm dark or neutral), while

predominantly white/purplish regions are star-forming (warm, high column density, and

8µm bright). Red/yellow regions are warm but have low column densities. The ‘generous’

HiSD region identifications (N(H2) > 0.4 × 1022 cm−2, 3σ temperature distribution) are

depicted on the left while the ‘conservative’ HiSD region identifications (N(H2) > 0.8 ×1022 cm−2, 2σ temperature distribution) are depicted on the right. Both identifications yield

relative lifetimes of approximately 70% starless and 30% star-forming, though the associated

relative maser lifetime is 2% on the left and 4% on the right. The sharp edges in red and blue

are the ‘source masks’ in the temperature and column density maps (see Battersby et al.

2011, for details).

– 8 –

30.300 30.200 30.100 30.000 29.900 29.800

0.1

00

0.0

00

-0.1

00

-0.2

00

-0.3

00

-0.4

00

Galactic longitude

Ga

lact

ic l

ati

tud

e

Generous

30.300 30.200 30.100 30.000 29.900 29.800

0.1

00

0.0

00

-0.1

00

-0.2

00

-0.3

00

-0.4

00

Galactic longitude

Ga

lact

ic l

ati

tud

e

Conservative

Fig. 3.— Depiction of starless (white contour), star-forming (black contour), and maser

(green contour) associated HiSD regions plotted on a three-color image in which red is the

temperature, blue is the column density, and green is the 8 µm emission. Predominantly blue

regions in this map are starless (cold, high column density, and 8 µm dark or neutral), while

predominantly white/purplish regions are star-forming (warm, high column density, and

8µm bright). Red/yellow regions are warm but have low column densities. The ‘generous’

HiSD region identifications (N(H2) > 0.4 × 1022 cm−2, 3σ temperature distribution) are

depicted on the left while the ‘conservative’ HiSD region identifications (N(H2) > 0.8 ×1022 cm−2, 2σ temperature distribution) are depicted on the right. Both identifications yield

relative lifetimes of approximately 70% starless and 30% star-forming, though the associated

relative maser lifetime is 2% on the left and 4% on the right. The sharp edges in red and blue

are the ‘source masks’ in the temperature and column density maps (see Battersby et al.

2011, for details).

– 9 –

will form a high-mass star. iii) The HiSD regions lifetimes don’t depend on mass. iv) The

star formation rate is constant as a function of time. v) Starless and star-forming regions

occupy similar areas on the sky (number of pixels). vi) The signatures at 8 µm (mid-IR

bright or dark) and their associated temperature distributions are good indicators of the

presence or absence of a high-mass star. vii) All pixels above the threshold column density

are beam-diluted dense cores and none are trace beam-filling low-surface density gas. While

these assumptions are generally reasonable, many are highly uncertain.

Assumption (i) is reasonable if there has been no large-scale triggering event. How-

ever,the entire study field lies in the densest part of the Scutum arm. The major HII regions

such as W43 may have been triggered by older generations of stars, and in turn may be

triggering the current populations of forming young massive stars and clusters at its periph-

ery. However, the conditions in this field may be typical of a spiral arm environment. The

argument for assumption (ii) presented above suggests that the majority of HiSD regions

(with characteristic density profiles and at typical distances) have the ability to form high-

mass stars, but there are always exceptions and outliers which will break this assumption.

This assumption could be improved with distance (and hence mass and size) determina-

tions. Assumption (iii) is necessary at this time, but could potentially be removed by a

careful separation of sources into mass bins when distances are determined. Assumption

(iv) is reasonable over a sufficiently large sample (similar to assumption (i)) and over these

relatively short Myr timescales. Since the column density threshold is the same for starless

and star-forming HiSD regions, and there is little variation above that threshold, both should

represent equal regions capable of forming high-mass stars, meaning that assumption (v) is

reasonable. Various studies (e.g., Battersby et al. 2010; Chambers et al. 2009; Rathborne

et al. 2006) argue in favor of assumption (vi), but more sensitive and higher resolution studies

will continue to shed light on the validity of this assumption.

3.2. Observed Relative Lifetimes

In both the ‘conservative’ and ‘generous’ cases, the relative fraction of pixels in the

starless phase (percent of total pixels) is 70% vs. the relative fraction in the star-forming

phase of 30%. These percentages are robust over a range in the cutoffs as shown in Figure

1. Slight changes in the temperature distributions (for example, including all HiSD regions

down to 0 K in the starless and up to 100 K in the star-forming case) also have a negligible

effect . Under the assumptions discussed in §3.1, high-mass star-forming clumps spend about

70% of their lives in the starless phase and 30% in the actively star-forming phase.

– 10 –

3.3. Maser association and Lifetimes

An absolute lifetime for the starless and star forming phases can be anchored to the

duration of the 6.7 GHz Class II CH3OH masers estimated of have lifetimes of ∼35,000 years

(van der Walt 2005) using the same unbiased surveys used to identify the maser locations

(Szymczak et al. 2002; Ellingsen 1996). This estimate is based on extrapolating the number

of masers detected in these surveys to a Milky Way total and using an IMF and global Milky

Way star formation rate to estimate the lifetime of the masers observed in these surveys, a

method similar to that used by Tackenberg et al. (2012) to determine the absolute lifetimes of

starless clumps. This method includes the fact that not every high-mass star-forming region

will necessarily go through a maser phase. The absolute lifetime derived using CH3OH

masers is controversial, but will likely be improved upon in future studies (e.g., the 6 GHz

multi beam maser survey, Green et al. 2009).

While the fraction of starless vs. star-forming HiSD regions is insensitive to the column

density cuts, the fraction of HiSD regions associated with methanol masers increases as a

function of column density (see Figure 1). In the ‘generous’ and ‘conservative’ cuts, the

methanol maser fraction is 2% and 4%, respectively. Therefore, in the ‘generous’ identifica-

tion, the ‘starless’ lifetime is 1.2 Myr and the ‘star-forming’ lifetime is 0.5 Myr for a total

dust clump lifetime of 1.7 Myr. In the ‘conservative’ case, the ‘starless’ lifetime is 0.6 Myr

while the ‘star-forming’ lifetime is 0.3 Myr for a total dust clump lifetime of 0.9 Myr. These

starless lifetimes correspond to roughly a free fall time for typical clump sizes and densities.

The 6.7 GHz CH3OH masers are nearly always found near the intersection of starless

and star-forming HiSD region distributions. The 6.7 GHz CH3OH masers exist for a short

time right when high-mass stars turn on.

3.4. UCHII Region Association and Lifetimes

The HiSD regions tend to be associated with UCHII regions as reported by Wood &

Churchwell (1989b). Wood & Churchwell (1989b,a), determine that the lifetimes of UCHII

regions are longer than expected based on the expected expansion ration of D-type ionization

fronts as HII regions evolve toward pressure equilibrium. They estimate that O stars spend

about 10-20% of their main-sequence lifetime indie molecular clouds as UCHII regions. If we

assume an O6 star, the main sequence lifetime is about 2.4 × 106 years (Maeder & Meynet

1987). If the star spends 15% of this lifetime in the UCHII region phase, the corresponding

lifetime of a UCHII region is about 3.6 × 105 years.

The remaining link between the absolute and relative lifetimes is the fraction of “starry”

– 11 –

pixels associated with UCHII regions, particularly O stars. While recent studies (e.g. An-

derson et al. 2011) show more complete samples of HII regions, Wood & Churchwell (1989b)

look for only the brightest UCHII regions, dense regions containing massive stars. More

sensitive studies (e.g. Anderson et al. 2011) show HII regions over wider evolutionary stages,

after much of the dense gas cocoon has been dispersed. Wood & Churchwell (1989b) searched

3 regions in the l = 30◦ field for UCHII regions finding them toward 2. These three regions

were all classified as “starry” in our study. The Wood & Churchwell (1989b) survey found

UCHII regions toward 2/3 “starry” regions surveyed. Since the 8 µm emission is indicative

of UV excitation of PAH molecules, the “starry” pixels show warmer dust temperatures, and

Bania et al. (2010) show that nearly all GLIMPSE bubbles are associated with UCHII re-

gions, we estimate that 50-100% of our “starry” pixels are associated with an UCHII region.

This corresponds to total lifetimes of 2.4 Myr (50% of “starry” pixels have UCHII regions)

and 1.2 Myr (100% of “starry” pixels have UCHII regions). The absolute lifetimes of the

starless phase then would be 0.8 - 1.7 Myr and the “starry” phase would be 0.4 - 0.7 Myr.

3.5. Comparison with Other Lifetime Estimates

Previous lifetime estimates, based primarily on mid-IR emission signatures at 24 µm to-

ward samples of IRDCs, found relative starless fractions between about 30-80% and extrap-

olate these to absolute starless lifetimes ranging from 103 - 104 years and up to 3.7 × 105

years. Chambers et al. (2009) using a sample of 106 IRDC cores find 65% starless and 35%

with 24 µm emission and EGOs (or 82% starless and 18% star-forming if only those cores

which contain 8 µm emission are considered to be star-forming). They extrapolate this to

an absolute starless lifetime of 3.7 × 105 years assuming a representative YSO accretion

timescale of 2 × 105 years (Zinnecker & Yorke 2007) for the star-forming phase. Miettinen

(2012) found a starless / star-forming ratio of 44% to 56% using LABOCA, and extrapolate

this in the same way to an absolute lifetime of 1.6 × 105 years. Wilcock et al. (2012) using

Hi-GAL find 18% starless, 15% with emission at 24 µm, and 67% with emission at 8 µm to

derive an absolute starless lifetime of 2 × 105 years. Tackenberg et al. (2012) target their

search toward starless clumps in the ATLASGAL survey and derive a lifetime of the starless

phase for the most high-mass clumps of 6 × 104 years based on an extrapolated total number

of starless clumps in the Milky Way and a Galactic star formation rate. Parsons et al. (2009)

derive 33% starless and 67% with 24 µm emission using SCUBA targeted toward IRDCs,

finding a starless lifetime of 103 to 104 years. Dunham et al. (2011) looked for mid-IR star

formation signatures toward Bolocam Galactic Plane Survey (BGPS Aguirre et al. 2011)

clumps and found that 56% are starless, or when accounting for chance alignment, 80% are

starless. Peretto & Fuller (2009) found a starless fraction between 80%- 32% toward IRDCs,

– 12 –

based on their lack of association with 24 µm point sources. Combining chemical models

with observations toward 59 high-mass star-forming regions, Gerner et al. (2014) found an

IRDC lifetime of 104 years, 6 and 4 × 104 years for high-mass protostellar objects and hot

molecular core phases respectively, and 104 years for the UCHII region stage.

Many previous lifetime estimates for high-mass star forming regions targeted IRDCs

and used emission at 24 µm as the indicator of star formation. Our analysis includes all

HiSD regions above a column density threshold. 8 µm emission is used to indicate high-mass

star formation; 24 µm emission may turn on earlier (e.g., Battersby et al. 2010) but may

not indicate high-mass star formation since it does not require UV-excited PAH emission.

Previous analyses calculated relative fractions of ‘clumps’ or ‘cores,’ defined in various ways

and with arbitrary sizes. Each ‘clump’ or ‘core’ is denoted as starless or star-forming; a single

24 µm point source would classify the entire clump as star-forming. In this study, individual

pixels are used. Therefore a higher starless fraction is not surprising. Maser lifetimes anchor

the absolute lifetimes. If a star-forming lifetime of 2 × 105 years is used, (representative

YSO accretion timescale, Zinnecker & Yorke 2007), the starless lifetime is reduced to 4.7 ×105 years.

4. Conclusion

Using column densities and dust temperature distributions derived from Hi-GAL, Spitzer

8 µm star formation signatures, and unbiased surveys for 6.7 GHz Class II CH3OH masers,

the relative lifetimes of the starless and star-forming phases for high-mass star-forming re-

gions in a 2◦ × 2◦ field centered at [`, b] = [30◦, 0◦] are determined. HiSD regions capable of

forming high-mass stars are identified by their large column density in the dust continuum.

They spend about 70% of their lifetimes in the starless phase and 30% in the star-forming

phase or embedded. ‘Starless’ refers only to a lack of high-mass stars and was determined

by the temperature (cold, roughly ≤ 25 K, see §2.1 for details) and signature at 8 µm (dark

or neutral). ‘Star-forming’ refers to active high-mass star formation as indicated by warmer

dust temperatures (> 25 K, roughly, see §2.1 for details) and emission at 8 µm. This relative

fraction is determined robustly over a variety of reasonable cutoff parameters.

Absolute lifetimes for the two phases are ached to the duration of methanol masers

(35,000 years) determined from van der Walt (2005). Two column density cutoffs suggest

a starless lifetimes of 0.6 to 1.2 Myr (70%) and a star-forming lifetimes of 0.3 to 0.5 Myr

(30%) for high-mass star-forming regions identified in the dust continuum. These starless

lifetimes correspond to roughly a free fall times at typical clump densities (high density cores

embedded in these clumps will have much shorter free fall times). Using an estimate of the

– 13 –

absolute lifetimes for UCHII regions and estimating their association with regions defined as

star-forming, we find a starless lifetime of 0.8-1.7 Myr (70%) and a star-forming lifetime of

0.4-0.7 Myr (30%). Together, these methods of anchoring the relative lifetimes to absolute

lifetimes produce similar results with a starless lifetime ranging from 0.6-1.7 Myr (70%) and

the star-forming lifetime of molecular clumps lasting about 0.3-0.7 Myr (30%). The 6.7 GHz

CH3OH masers appear at the intersection between starless and star-forming HiSD regions.

This indicates that these masers exist for a short period of time as a high-mass star turns

on, and that 6.7 GHz masers trace the earliest phase of high-mass star formation.

We thank H. Beuther, J. Tackenberg, A. Ginsburg, and J. Tan (and others?? XXXX) for

helpful conversations regarding this work. This work has made use of ds9 and the Goddard

Space Flight Centers IDL Astronomy Library. Data processing and map production of the

Herschel data has been possible thanks to generous support from the Italian Space Agency via

contract I/038/080/0. Data presented in this paper were also analyzed using The Herschel

interactive processing environment (HIPE), a joint development by the Herschel Science

Ground Segment Consortium, consisting of ESA, the NASA Herschel Science Center, and

the HIFI, PACS, and SPIRE consortia. This work was supported by NASA through an

award issued by JPL/Caltech via NASA Grant #1350780.

REFERENCES

Aguirre, J. E., Ginsburg, A. G., Dunham, M. K., et al. 2011, ApJS, 192, 4

Anderson, L. D., Bania, T. M., Balser, D. S., & Rood, R. T. 2011, ApJS, 194, 32

Bania, T. M., Anderson, L. D., Balser, D. S., & Rood, R. T. 2010, ApJ, 718, L106

Battersby, C., Bally, J., Jackson, J. M., et al. 2010, ApJ, 721, 222

Battersby, C., Bally, J., Ginsburg, A., et al. 2011, A&A, 535, A128

Benjamin, R. A., Churchwell, E., Babler, B. L., et al. 2003, PASP, 115, 953

Beuther, H., Walsh, A. J., Thorwirth, S., et al. 2007, A&A, 466, 989

Carlhoff, P., Nguyen Luong, Q., Schilke, P., et al. 2013, A&A, 560, A24

Chambers, E. T., Jackson, J. M., Rathborne, J. M., & Simon, R. 2009, ApJS, 181, 360

Cyganowski, C. J., Brogan, C. L., Hunter, T. R., & Churchwell, E. 2011a, ApJ, 743, 56

– 14 –

Cyganowski, C. J., Brogan, C. L., Hunter, T. R., Churchwell, E., & Zhang, Q. 2011b, ApJ,

729, 124

Cyganowski, C. J., Whitney, B. A., Holden, E., et al. 2008, AJ, 136, 2391

Dunham, M. K., Robitaille, T. P., Evans, II, N. J., et al. 2011, ApJ, 731, 90

Egan, M. P., Shipman, R. F., Price, S. D., et al. 1998, ApJ, 494, L199

Ellingsen, S. 1996, PhD thesis, Physics Department, University of Tasmania, GPO Box

252C, Hobart 7001, Australia

Elmegreen, B. G. 2000, ApJ, 530, 277

—. 2007, ApJ, 668, 1064

Gerner, T., Beuther, H., Semenov, D., et al. 2014, A&A, 563, A97

Green, J. A., Caswell, J. L., Fuller, G. A., et al. 2009, MNRAS, 392, 783

Hartmann, L., & Burkert, A. 2007, ApJ, 654, 988

Kauffmann, J., & Pillai, T. 2010, ApJ, 723, L7

Krumholz, M. R., & McKee, C. F. 2008, Nature, 451, 1082

Lada, C. J., & Lada, E. A. 2003, ARA&A, 41, 57

Maeder, A., & Meynet, G. 1987, A&A, 182, 243

Miettinen, O. 2012, A&A, 542, A101

Minier, V., Conway, J. E., & Booth, R. S. 2001, A&A, 369, 278

Molinari, S., Swinyard, B., Bally, J., et al. 2010, A&A, 518, L100

Mueller, K. E., Shirley, Y. L., Evans, II, N. J., & Jacobson, H. R. 2002, ApJS, 143, 469

Myers, P. C. 2009, ApJ, 700, 1609

Omont, A., Gilmore, G. F., Alard, C., et al. 2003, A&A, 403, 975

Parsons, H., Thompson, M. A., & Chrysostomou, A. 2009, MNRAS, 399, 1506

Perault, M., Omont, A., Simon, G., et al. 1996, A&A, 315, L165

Peretto, N., & Fuller, G. A. 2009, A&A, 505, 405

– 15 –

Pestalozzi, M. R., Minier, V., & Booth, R. S. 2005, A&A, 432, 737

Pillai, T., Wyrowski, F., Carey, S. J., & Menten, K. M. 2006, A&A, 450, 569

Rathborne, J. M., Jackson, J. M., & Simon, R. 2006, ApJ, 641, 389

Schuller, F., Menten, K. M., Contreras, Y., et al. 2009, A&A, 504, 415

Szymczak, M., Kus, A. J., Hrynek, G., Kepa, A., & Pazderski, E. 2002, A&A, 392, 277

Tackenberg, J., Beuther, H., Henning, T., et al. 2012, A&A, 540, A113

Tan, J. C., Krumholz, M. R., & McKee, C. F. 2006, ApJ, 641, L121

van der Walt, J. 2005, MNRAS, 360, 153

Walsh, A. J., Burton, M. G., Hyland, A. R., & Robinson, G. 1998, MNRAS, 301, 640

Wilcock, L. A., Ward-Thompson, D., Kirk, J. M., et al. 2012, MNRAS, 422, 1071

Wood, D. O. S., & Churchwell, E. 1989a, ApJ, 340, 265

—. 1989b, ApJS, 69, 831

Zhang, B., Moscadelli, L., Sato, M., et al. 2014, ApJ, 781, 89

Zinnecker, H., & Yorke, H. W. 2007, ARA&A, 45, 481

This preprint was prepared with the AAS LATEX macros v5.2.