Star Formation in the Local Group

Phil MasseyLowell ObservatoryStromfest April 2008

Disclaimer

Michael sent around this list of “influential papers” by the Stroms. That’s like saying that a tornado made it cloudy. It’s true, but also somewhat irrelevant.

“Tell a good story.”

Telling the story

When stars form in an OB association or cluster, what is the range of slopes of the initial mass function?

Do stars form coevally or is there a progression with mass?

What impact do massive stars have on their environment?

How are the answers to the above affected by metallicity?

Emphasis here will be on Massive Stars

Need to be able to see them at large distances!

Short lifetimes (3-10Myr) means that you get an “instantaneous snapshot” of star formation---they don’t accumulate for billion of years in the field. (No emeritus status for massive stars.)

Massive Stars are Extremely Rare

For every 20 Mo in the Milky Way, there are roughly 10 solar-type stars

For every 100 Mo star, there are over 10 solar-type stars.

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But they exert a large influence

Provide most of the UV flux in a galaxy, causing HII regions and hence delineating the arms in spiral galaxies.

Their strong stellar winds, and eventual disruption as SNe, trigger new generations of star formation.

They manufacture most of the C, N, and O in the universe.

Properties of Massive Stars

m≈15-150Mo

L≈10 to 10 Lo

Teff≈30,000 K to 50,000 K on the main-sequence

τ≈3-10 Myr

M-dot ≈10 to 10 Mo/yr for O stars.

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Oddly, the highest mass stars are not the

brightest in a galaxy

Stellar evolution occurs at nearly constant Mbol but BC a strong function of effective temperature!

The Effect of the BC!

A young 85Mo O-type star will be 15x fainter

than a 25Mo A-type

supergiant!

From Massey et al (1995)

The Effect of the BC!

Missing for a reason!

\

From Massey et al (1995)

TechniquesNeed UBV photometry AND optical spectroscopy:"

Photometry:

UBV will give reddening-free index Q, allowing one to select the intrinsically blue stars for followup spectroscopy.

U-B, B-V two-color plot will allow us to separate foreground/background from cluster members.

Optical spectroscopy:

Yields direct measure of E(B-V), allowing Av to be determined.

For Galactic clusters, determines the distance via spectroscopic parallax.

Determines Teff and the all-important bolometric correction, leading to placement on the HRD!

TechniquesNeed UBV photometry AND optical spectroscopy:"

Can’t you just do this from photometry?

Works OK for stars for intermediate-mass stars but not for core-H burning massive stars:

L∝ m (M-L is pretty flat up at the top)∆log m = -0.2×∆Mbol∆Mbol = -6.8×∆log Teff

From 50,000 K to 35,000 K (∆log Teff=0.155) Q changes by 0.03 mag; i.e. ∆log Teff=5.2×∆Q, so ∆log m = -7.0∆Q.

An error of 0.1 in Q is an error of 0.7 in log m, i.e., a factor of 5 in mass (20Mo vs 100Mo!)

2.0

Or to put it another way...

Text

Too far down on

the Rayleigh-

Jeans tail...

from Conti (1986)

This Is a Wonderful Time!

Improved stellar models:

Stellar evolutionary models are increasingly sophisticated, with improved physics.

Highly accessible thanks to the kindness of the Geneva and Padova teams and the existence of the Internet.

This Is a Wonderful Time!

Improved observational capabilities:

Mosaic CCD cameras on 4-m telescopes (LG Survey produced UBVRI photometry of half a million stars in 9 galaxies).

High through-put spectrographs on large telescopes (Blue Channel+Hectospec on the MMT, GMOS on Gemini).

UV and high spatial resolution of HST about to be restored!

Why Study Them in the Galaxies of the Local Group?

NGC 6822, from the Local Group Galaxies Survey

Metallicity (Z) varies by a factor of ≈20 amongst the galaxies currently actively forming stars (WLM: 1/10th solar; M31: 2x solar).

Why Study Them in the Galaxies of the Local Group?

Foreground contamination can be severe.

But there are challenges...

Foreground contamination can be severe

From Massey et al (2007, AJ, 133, 2393)

But, in some cases we can use 2-color diagrams to help

us out...

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Impacts on their environments

Wind-blow bubble

N44F (Naze & Chu)

But, there may be other, lesser understood effects

Red supergiants lose dust at the rate of 10 to 10 Mo/yr. If you do the math, this turns out to be 3x10 Mo/yr/kpc in the solar neighborhood. That’s only about 0.5% of what AGBs contribute.

BUT: in a galaxy at large look back time, or a metal-poor starburst, RSGs should DOMINATE dust production.

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HDF

So, What Have We Learned? Star Formation, the IMF, and

the upper-mass limit

Magellanic Clouds and the Milky

Way

Image by Roger Smith/NOAO/AURA/NSF

Stories of Star Formation

Detailed spectroscopic and photometric studies of young clusters reveal very different histories of star formation.

Stories of Star Formation

Herbig (1962, ApJ, 135, 736) first suggested that low and intermediate mass stars might form over a prolonged time in a cluster, followed by the formation of high mass stars, which halts all star formation. Not all clusters fit this model....

Back before HST made M16 famous...

Hillenbrand et al (1993, AJ, 106, 1906) studied the stellar content of the cluster, and found that there were intermediate-mass stars with ages as young as few hundred thousand years. Yet the massive star population had an age of 2 million years.

“...Thus the formation of O stars neither ushered in nor concluded the star-formation process in this young

complex.”

On the other hand...

R136 in the LMC

FOV

13”x13”

= 3.5x3.5 pc

WFPC1 team

R136: “super star cluster” in 30 Dor in the

LMCHST photometry and spectroscopy revealed

intermediate-mass population began forming 6 million years ago, and stopped about 2 million years ago. There is also a very large number of extremely massive stars (the highest mass stars known!) that formed 1-2 million years ago.

Two stories of Star Formation...

In very rich, dense clusters (such as R136), the formation of intermediate-mass stars is stopped shortly after the formation of high-mass stars, due to the effects of stellar winds on the surrounding gas.

In less rich clusters (such as M16), production of intermediate-mass stars is not halted until the first Wolf-Rayet stars are produced (with their super strong winds), or possibly even the first supernovae.

The Initial Mass Function of Massive Stars

Studies of OB associations and clusters in the Milky Way and Magellanic Clouds show no evidence for any effect with metallicity. Variations that are seen are observational and/or statistical (Massey 1998, Kroupa 2001).

IMF Slopes for OB Associations

Γ is approximately Salpeter over a factor of 4 4 in metallicity and 200 in stellar density

ΓΓ

IMF not a function of metallicity?

This is somewhat surprising given that metals provide the primary cooling mechanism in molecular clouds, and hence cloud temperatures (and thus the Jeans mass) should depend upon the metallicity (Larson 1998). But so far have only probed a small range in metallicity!

What’s next?

M31 with 2x solar metallcity

NGC 206the “h and Chi Per”

of M31!

What’s next?

Would be nice to know the star-forming history and IMF of NGC 206 in M31 at 2x solar metallicity. Could be done with Gemini/GMOS.

WLM: lowest metallicity of any star-forming LG galaxy (1/10th solar)

What’s next?Would be nice to know the star-forming history and IMF of OB associations in WLM, the lowest metallicity star-forming galaxy in the Local Group.

The upper-mass limit

R136 taught us another important lesson:

The highest mass stars in R136 were an (unprecedented) 150 Mo. However, this is just what we would have expected from extrapolating the intermediate-mass IMF: it’s just where the IMF would peter down to a single star.

Upper-mass “limits” are so far statistical, not physical. Whatever it is that limits the ultimate mass of a star, we have yet to encounter it in nature.

Of course, not everyone agrees with that...

Oey & Clarke (2005) use Massey et al. (1995)’s OB association data to argue that there are no stars with masses greater than 120-200Mo in the local universe.

However, the sample includes clusters with ages more than a few million years (Massey 1998), and stars of masses 200Mo are not expected to last more than 2-3 Myr.

Of course, not everyone agrees with that...

Weidner & Droupa (2004) and Koen (2006) have extrapolated the intermediate-mass IMF of the R136 cluster to argue that there should be more stars with inferred masses >120Mo than there are.

However, the observational work (Massey & Hunter 1998) used a very conservative method to estimate the masses for the highest mass stars, and we assumed a single Teff (and hence BC!) for all O3s! stars.

Of course, not everyone agrees with that...

Best evidence to date for evidence of an upper mass cutoff is from the Figer (2005) K-band luminosity function of the Arches cluster, which seems to be truncated. There are no stars more luminous than 130Mo, while extrapolating the IMF suggests there should be 18.

But, conversion of K-band luminosity to mass is untested. Why not repeat this for some cluster where the answer is partially known, such as R136?

This could be answered with better data

Go after the 30-40 hottest stars in R136 with HST/STIS and get good enough S/N to model the stars---get real Teff and Mbols. Would take 100-200 orbits.

Pilot program submitted for N3603.

Summary

When stars form in an OB association or cluster, what is the range of slopes of the initial mass function?

So far, Salpeter, but limited z-range probed.

Do stars form coevally or is there a progression with mass?

It depends!

Summary

What impact do massive stars have on their environment?

More than just wind-blown bubbles!

How are the answers to the above affected by metallicity?

So far, not at all, but only 4x has been probed. Could extend this to 20x.

So, thanks!

But let’s not stop there!

Plenty of fun still to be had!

Moonrise over the Andes