MYStIX: New Insights into Clustered Star...

Eric Feigelsonwith Michael Kuhn, Konstantin Getman,

and the MYStIX Team

Penn State University

Natl Astro Obs China, Beijing

July 2018

MYStIX: New Insights into Clustered Star Formation

Most stars form in clusters with 102-104 members (105 in starburst galaxies) within giant molecular cloud complexes.

Most clusters become gravitationally unbound when the natal molecular gas dissipates, and members become field stars like the Sun.

Often the most massive members are OB stars that ionize the local interstellar medium (H II regions); their ionization fronts, stellar winds and supernova ejecta can both inhibit and trigger star formation in the nearby cloud material (‘feedback’).

Motivation for MYStIX

Eric Feigelson 2018 2

Although there are extensive observations and theory about the formation of single/binary stars, our understanding of

cluster formation is relatively weak

– Do rich clusters form monolithically or by mergers of subclusters?– Do rich clusters form quickly or slowly (`age spreads’ in HR diagrams)?– When and how does cluster expansion occur?

Other questions not covered in this talk … – Are 2/3-body dynamical interactions important (`runaway stars’)?– Is the stellar Initial Mass Function universal, and why?– Is `mass segregation’ universal, and why does it occur? – Is triggering on HII regions peripheries an important SF mode?– How long does SF proceed in a GMC?– Are HII regions hostile environments for protoplanetary disks?– What is the source of the Galactic hot interstellar medium?

We suggest that progress in understanding is inhibited by poor characterization of the clustered stellar population:

membership, ages, masses, kinematicsEric Feigelson 2018 3

The Orion Clouds: Nearest massive star forming region

Orion Nebula ClusterOB-dominated star clusterN~3000 stars, Age ~1-2 Myr

Becklin-Neugebauersmall cluster of protostarsembedded in OMC-1

Protoplanetary disks around Orion stars

4-5 Myr

1-2 Myr

0 Myr

Alves & Bouy 2012

Multiple star clusters with age spreads/gradients

4 Myr

10 pc

ONC

5

Beyond d~1 kpc, optical/infrared images have great difficulty identifying cluster members due to field star contamination.

This is a 2MASS near-IR image of the NGC 6357 complex …

Can you find the three

rich young clusters?


This is an optical image of the Flame Nebula in Orion … can you find the cluster members within the confusing nebulosity and obscuration of the surrounding cloud?


But Chandra X-ray Observatory sees the Flame Nebula members clearly!

Spitzer Space Telescope is badly contaminated by the heated PAH dust

Pre-main sequence stars produce variable X-ray emission from enhanced magnetic reconnection flares: ~102 more powerful ~102

more often than the active Sun.

X-ray selection complements more common Ha and infrared-excess selection for PMS stars. Together they give Class I-II-III populations. X-rays also capture OB stars, and can reach AV~10-200. Removing X-ray sources reveals diffuse X-ray plasma from shocked OB winds.

Chandra detects 102-103 in massive SFRs at distances of 1-few kpc in ~1 day exposures. During the mission, we increasingly realized that X-ray images were useful for improving the census of pre-main sequence (PMS) stellar populations.

Chandra field of view is 17’x17’; most effective in star forming regions at distances around 0.4-3 kpc. Mosaics needed for mesoscale (10-50 pc) scales.

The X-ray Perspective


MYStIXMassive Young Stellar complex

study in Infrared and X-ray

Multiwavelength archive survey of 20 massive SFRs

Data analysis: 2009-12Papers: 2013-18

http://astro.psu.edu/mystix

Now being combined with SFiNCsStar Formation in Nearby Clouds

Led at Penn State by Eric Feigelson, Leisa Townsley & Kostantin Getman

with Michael Kuhn, Patrick Broos, Matthew Povich, Alexander Richert, Tim Naylor, and others


http://astro.psu.edu/mystix

The MYStIX Approach

Premise: Astrophysical uncertainty is due to the paucity of stellar data

Most rich clusters lie >1 kpc away with bad obscuration, nebular emission (HII region), contamination by Galactic field stars. Traditional optical and IR methods give small & biased census.

The MYStIX solution: Obtain better stellar census by combining high-sensitivity selection of X-ray, IR-excess and OB stars

– Chandra X-ray Observatory (ACIS-I)

– UKIRT/UKIDSS (WFCAM)

– Spitzer Space Telescope (IRAC)

Emphasize high-sensitivity and uniform analysis

Sample: 20 OB-dominated star forming regions at 1<d<4 kpc

SFiNCs uses MYStIX methods for 19 regions d<1 kpc w/o O stars


The MYStIX Sample

12

Credit: NASA/JPL-Caltech/R.Hurt

Location of MYStIX star forming regions

MYSTiX: Overcoming practical challenges• Chandra ACIS analysis of fields with ~1000 point sources and diffuse emission

ACIS Extract software package (Kuhn et al. 2013a, Townsley et al. 2014)

• Spitzer analysis of crowded fields & complex PAH nebular emission

Improved IRAC Team pipeline (Kuhn et al. 2013b)

• UKIRT analysis of crowded fields

Improved UKIDSS pipeline (King et al. 2013)

• Confusion in X-ray/infrared matching

Improved probabilistic magnitude-weighted matching (Naylor et al. 2013)

• Contamination in infrared excess sample

Improved IRAC selection criteria (Povich et al. 2013)

• Combine heterogeneous X-ray/infrared criteria to get a unified sample

Naïve Bayes classifier (Broos et al. 2013)

Result: Sample of 31,474 MYStIX Probable Complex Members

in 20 massive star forming regions (Feigelson et al. 2013, Broos et al. 2013)

All MYStIX papers and tables available at http://www.astro.psu.edu/mystix 14

The Chandra Data

Classifier results:Black = MPCMs Green = AGNs Blue = Unclassified Red = Field stars


Yellow = X-ray selected

Red = MIRES

Cyan = Published OB

MPCMs: MYStIX Probable Complex Members

MPCM catalog:Broos et al.2013

16

MYStIX science results reviewed here …

– Diverse spatial distributions of young stars in SFRs (Michael Kuhn, 2014-15)

– New PMS star age estimator based on X-ray/IR photometry. Unexpected age gradient (KostantinGetman, 2014)

– Expansion of clusters (Kuhn et al. 2016, 2018)


Additional MYStIX/SFiNCs topics …

– Diffuse X-ray emission in SFRs: Birth of the Galactic hot interstellar medium (Townsley)

– No evidence for protoplanetary disk destruction in HII regions (Richert)

– Improved disk longevity distribution (Richert)

– Lists of new embedded protostars & OB stars(Romine, Povich)


Apparent star surface density maps(partially corrected for X-ray sensitivity variations)

30 pc


ε = 1 – a/b

rc = (a2 + b2)1/2

(α0,δ0)

φ

Defining star clusters:Finite Mixture Model

1. Cluster properties determined through maximum likelihood estimation (MLE) assuming isothermal ellipsoidal shape

2. Number of clusters determined with MLE model selection criteria

3. Cluster membership determined with secondary decision rules

k = 5 subclusters

Kuhn et al. 2014a20

Modeling the ONC region with 4 isothermal ellipsoids

Green = X-ray selectedRed = IR-excess selected


2

16

57

128

226

353

507

689

901

1139

1405

30 9:00:00 30 8:59:00 58:30

20

25

-47:30

35

40

R ight ascension

Declination

5 pc

20

32

68

130

215

326

460

619

804

1011

1243

30 20 10 5:42:00 50 40 30 20 41:10

44

46

48

-1:50

52

54

56

58

-2:00

02

04

R ight ascension

Declination

1 pc

Flame Nebula M 17RCW 38

core

halomulti-modalclumpy structure

simple

Kuhn et al. 2014a


The NGC 6357 complex

2,235 MPCM starsin 6 (sub)clusters

Recall JHK image …

Heuristic Morphological Classifications

Kuhn et al. 2014a


The observed surface density of young

stars in MPCM maps depend on the

happenstance of Chandra exposures,

region distance & absorption, and off-axis angle.

Kuhn et al. (2014b) correct for these effects, assuming the X-ray luminosity functions have the same shape as the ONC XLF. They derive intrinsic star surface density maps.

Astrophysical quantities can now be obtained for the 142 ellipsoidal subclusters: core radii (pc), central star density (stars/pc3), crossing/relaxation times (Myr), and AgeJX (Myr).

From samples to populations …


MYStIX surface density fully corrected for X-ray incompleteness

• Regions shown on the same scales of intrinsic size and stellar surface density

• Range: 1–50 pc size, 1–30,000 stars / pc2

Kuhn, Getman &Feigelson, 2014b

Estimating Ages for Young Stars & ClustersL b

ol[L

O]

Dis

k Fr

acti

on

Age [Myr]

Teff [K]

Traditional methods: + HR diagram with pre-main

sequence isochrones+ Protoplanetary disk fraction


Problems with Pre-Main Sequence Ages(A&A Review: Preibisch 2013)

• Individual age estimates are highly uncertain

• Apparent HR diagram age spreads are hard to interpret

• Disk duration is astrophysically complex

• Calibration (absolute ages) is poor

• Optical/IR spectroscopic data are not available for most of the MYStIX stars --- the HRD methods are not applicable

A new age estimator was developed for MYStIXby Getman et al. 2014 a&b


0.5 Myr

5 Myr

from X-ray photometry

Bo

lom

etri

c lu

min

osi

ty

Mass [Mo]

MJ[m

ag]

AgeJX: Concept

Lx-Mass relationship is universal during the early PMS phase.

(Telleschi et al. 2007)Dashed lines:PMS isochrones (Siess et al. 2000)


8 um 500 um

Older subclustersare less absorbedthan younger subclusters

Getman et al. 2014a30

Age gradients across SFRs from AgeJXHerschel SPIRE 500um

31

• Our new age estimator (AgeJX) offers a number of advantages over the previous methods: ➢ sensitive to wide range of evolutionary stages➢ gives estimates of median ages for spatially distinct

subclusters.

• Application to the MYStIX ellipsoidal subclusters gives discovery of previously unknown age gradients across massive SFRs with spatially distinct SF episodes:

o embedded subclusters (AgeJX < 1 Myr)o revealed subclusters (AgeJX ~ 1-2 Myr)o distributed stars (AgeJX ~ 3-5 Myr)

Result

Getman et al. 2014aEric Feigelson 2018 32

Apply the same analysis within nearby rich clusters …CORE-HALO AGE GRADIENTS in NGC 2024 and ONC

NGC 2024

Orion Nebula ClusterValidation using K-band disk fractionGetman et al. 2014b 33

Core-halo age gradients appear in most young clusters(Getman, Feigelson & Kuhn 2018)

80% show age trends where stars in cluster cores are younger than in

outer regions. Observed gradient ~ 1 Myr / pc.


This is astrophysically interesting !

Cannot be explained by monolithic collapse models (where stars are

older in core) or standard filament infall models (where subclusters of

different ages are mixed by violent relaxation).

Requires combination of continued feeding of molecular material to

give late star formation in core, plus dispersion of older stars during

merging process.

This is nicely explained in hierarchical collapse hydrodynamical model

of Vazquez-Semadeni et al. (2017).


Possible explanations for younger cores

Getman et al. 2014b 36

Most young clusters must be unbound, releasing stars into the field.

Multivariate relationships among subclustersshow clear evidence that core radius increases (and core density decreases) with time.

Kuhn et al. 2015

Evidencefor clusterexpansion !! 37

But with the advent of ESA’s Gaia astrometric satellite, we now have direct evidence for expansion within individual clusters

(Example: NGC 6530 illuminating the Lagoon Nebula)

While it is difficult to see any pattern in the raw data …

… an organized velocity gradient is clearly present … 38

… that can be clearly attributed to a net ourward motion. 70% of clusters show ~0.5 km/s expansion.

Additionally, some clusters show a `Hubble flow’ indicating sorting of high-velocity stars in the outer regions.

Kuhn, Hillenbrand, Sills et al. 2018 (arXiv)Eric Feigelson 2018 39

The Gaia kinematics of MYStIX stars give some valuable insights into cluster dynamics

The velocity dispersion is Gaussian suggesting virial equilibrium …

… but the velocity dispersion exceeds the virial dispersion for most clusters, indicating expansion

The expansion velocity is typically ~half of the velocity dispersion

Kuhn, Hillenbrand, Sills et al. 2018 (arXiv)Eric Feigelson 2018 40

Conclusions from MYStIX (2018)

High-resolution X-ray, near- and mid-IR imaging together with advanced statistical methods provide a large, reliable census of young stellar population in massive star forming regions. Together with AgeJXestimates, XLF scaling to total stellar populations, Gaia kinematics, and new statistical methods, an empirical picture of star formation in giant molecular clouds is emerging from MYStIX:

Dense clusters form asynchronously in molecular cores, evacuate their molecular environment, and dynamically expand over millions of years. Subclusters are non-coeval; age spreads within & between clusters are real. Cluster structures and star formation histories are complex. Core-halo age gradients and cluster expansion measurements support astrophysical theories of hierarchical assembly of clusters from subclusters during filament infall, followed by equilibration and expansion to disperse stars into the Galaxy.


MYStIX: New Insights into Clustered Star...

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