NEARBY YOUNG STELLAR CLUSTERS: AN OVERVIEW OF IMF S AND AGE SPREADS - - PowerPoint PPT Presentation

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NEARBY YOUNG STELLAR CLUSTERS: AN OVERVIEW OF IMF S AND AGE SPREADS - - PowerPoint PPT Presentation

Aug 24, 2023 224 likes •628 views

NEARBY YOUNG STELLAR CLUSTERS: AN OVERVIEW OF IMF S AND AGE SPREADS LYNNE A. HILLENBRAND (CALTECH) I am only the messenger please no imf baggage or weapons! Abstract: This talk will summarize methods for measuring the masses and ages

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NEARBY YOUNG STELLAR CLUSTERS: AN OVERVIEW OF IMFS AND AGE SPREADS

LYNNE A. HILLENBRAND (CALTECH)

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Abstract: This talk will summarize methods for measuring the masses and ages of young stars, including the techniques and their challenges, then review recent literature results concerning mass and age distributions in clusters. Some longstanding problems apropos distances, cluster membership, spatial resolution, and sensitivity have been improving, while othersincluding systematics in assumed intrinsic photospheric propertiesand theoretical pre-main sequence evolutionary tracks, remain.

I am only the messenger – please no imf baggage or weapons!

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  • Salpeter – 1955
  • Miller and Scalo – 1979
  • Scalo – 1986
  • Kennicutt - 1998
  • Larson – 1998
  • Scalo – 1998
  • Kroupa – 2002
  • Chabrier – 2003
  • de Marchi – 2005
  • McKee, Ostriker – 2007
  • Yorke, Zinnecker – 2009
  • Elmegreen – 2009
  • Bastian – 2010
  • Parravano – 2011
  • Jeffries – 2012
  • Luhman - 2012
  • Kroupa – 2013
  • Offner – 2014
  • Maschberger – 2013
  • Krumholz – 2014

BR BROAD SUMMARIES OF THE IM IMF CO CONCE CEPT AND OB OBSERVE VED OR OR PROP OPOS OSED FUNCTION ONAL FO FORMS (E (ESPECIALLY APRO ROPOS YOUNG-ST STAR-SP SPECIFIC ISSU SSUES) S)

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WH WHAT ARE WE WE TRYI YING TO MEASURE?

Hopkins, 2018 Kouwenhoven et al.

dN(M)/dM α M-α dN(logM)/dlogM α MΓ

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SO SOME REALITIES S OF ANY IM IMF ST STUDY

  • We want to infer stellar mass, but we actually measure light!
  • Usually we measure as single point sources what are actually multiple star systems -
  • combined light that we naively convert to mass. It is not possible to distinguish

every individual object given the wide range of binary parameters.

  • For young clusters, inferring an IMF is completely dependent on assumptions about

the cluster age (and any age spread). Metallicity effects are not relevant.

  • Producing the IMF over the full mass spectrum requires a populous enough cluster

to have formed massive stars, and a young enough cluster to still have them.

  • The dynamic range from 20 Msun to 0.1 Msun is ~12 magnitudes in the red optical,

and from 0.1 Msun to 0.01 Msun is another ~7 magnitudes è 4x107 in total!

  • There is a wide range of measurement techniques (monochromatic, polychromatic,

spectroscopic) and a wide range of analysis techniques.

  • There is likely some stochasticity in the star formation process. Thus, due to finite

sampling, we might not expect the inferred IMF to be exactly the same everywhere – though it is clearly quite similar in many different types of star-forming environments.

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ST STARS S >100-500 500 MY MYR AR ARE RE RELATIVELY STRA RAIGHTFORW RWARD RD WH WHEN LOCATED IN CLUSTERS

  • Evolved stars are bright!
  • Isochrones are readily traced by

accurate/precise photometry.

  • Free parameters of distance and

reddening are uniform across the population, and can be solved for.

  • Some nuisance parameters like

metallicity and binaries.

  • Challenges: upper IMF depleted by

stellar evolution and lower IMF can suffer from dynamical evolution.

Meneses-Goytia et el., 2015

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ST STARS S <10 MY MYR OL OLD, BY CON ONTRAST, ARE A MESS, EV EVEN EN WHEN EN LOCATED ED IN CLUSTER ERS

  • Young stars are also relatively bright.
  • Empirical sequences are much more

diffuse due to several effects:

  • Some regions close enough that there is a

distance spread è enhanced dispersion in brightness / luminosity.

  • Extinction is differential and can be quite
  • large. Reddening can be non-standard.
  • Activity, especially accretion
  • Still “nuisance” parameters**such as
  • binaries. But no metallicity effects.

** well-determined binary parameters are of

course an important constraint on s.f. physics

Amard et el., 2019

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Hartmann, Herczeg, Calvet2016 Barensten et al. 2013 Increasing accretion rate

  • Line emission

complicates spectral typing.

  • Continuum excess

distorts colors.

OB OBSERVABLE CON ONSEQUENCES OF OF ACCRETION ON

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  • Spectral type from optical or

infrared spectra used to estimate photospheric temperature.

  • Accretion effects should be

taken into account during the spectral typing process.

  • De-reddened photometry plus

bolometric correction allows luminosity estimate.

1 um 10 um

BU BUILDING HR DIAG AGRAM AMS FOR YOUNG STAR ARS

[Robinson et al. 2019]

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A A CONTINUUM OF AC ACCRETION BU BURST BE BEHAVIOR

[Cody et al. 2017]

At 5-8 Myr, 14% of the objects with disks exhibit with these types of lightcurves

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[see also Ansdell2016 and Hedges 2018]

Cody and Hillenbrand (2018)

Aperiodic Examples Quasi-periodic Examples

AL ALSO A A CO CONTINUUM OF OF FA FADING/DIPPING BE BEHAVIOR

Δmag = 7!

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VA VARIABILITY TIME SC SCALES S AND AM AMPLITUDES (AT AT RELAT ATIVELY OLD AGES OF 5-8 8 MY MYR)

Cody and Hillenbrand (2018) Long = Bursters, Stochastics Aperiodic Dippers Int = Quasi-periodic Dippers Quasi-periodic Symm. Short = Periodic Multi-Periodic Amplitude ranges of most ..disk categories are similar. 0.1% 300% week month day 10%

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In Infrared ed Va Variability Too (Spitzer data)

~50% of identified variables also vary in color ~33% are periodic ~25% “dip” and ~25% “burst” ~50% “trend” over a month ~20% “stochastic” [ Rebull et al 2014, 2015 ]

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  • Young stars are active, including underlying spottedness plus any

superposed accretion effects, both of which cause blue-ingat short

  • wavelengths. Also dust/gas causing red excess at longer wavelengths.
  • Question of at what wavelengths we can measure mostly the stellar

photosphere (vs disk effects) and hence how to best determine

  • extinction correction to account for reddening
  • bolometric correction from measured flux to luminosity.
  • Complication of variability:
  • use median magnitude?
  • use bright state for dippers/faders?
  • use faint state for bursters?
  • Need to be thoughtful regarding techniques.

ST STARS S <10 MY MYR OL OLD, BY CON ONTRAST, ARE A MESS, EV EVEN EN WHEN EN LOCATED ED IN CLUSTER ERS

Median RMS values in the ONC: <0.19> mag at 0.8 um <0.14> mag at 1.2, 1.6, 2.2 um <0.07> mag at 3.6, 4.5 um Variability tail extends to >2 mag!

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Amard et al. 2019

PR PRE-MA MAIN SE SEQUENCE EV EVOLUT UTIONARY TR TRACKS ST STILL CA CARRY SY SYST STEMATICS S BE BETWEEN MO MODEL SETS

MODEL PHYSICS INPUTS “BIRTHLINE” EFFECTS

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ST STELLAR CONTRACTION TH THEORY

Despite improvements, pre-main sequence evolutionary tracks still do not reproduce cluster luminosity vs effective temperature sequences. This directly impacts IMF studies for all M < 1 Msun, especially M < 0.1 Msun.

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Inferred mass is age-dependent. Results can differ if simply interpolate HRD vs adopt a single age or age +/- sigma(age).

ST STELLAR CONTRACTION THEORY

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HO HOW ARE WE TRYING TO TO DO DO THE E MEA EASURING? (T (THE EXTRAGALACTIC WAY)

Meneses-Goytia et el., 2015

Integrated Light methods 10 Gyr 2 Gyr

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HO HOW ARE WE TRYING TO TO DO DO THE E MEA EASURING? (T (THE EXTRAGALACTIC WAY)

Dries et el. 2016

Attempt to distinguish metallicity, age, and the IMF from single spectra!

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HO HOW ARE WE TRYING TO TO DO DO THE MEASURING? (TH THE GALA LACTI TIC / NEARBY STAR FORMING REGIONS WA WAY)

Maia et el., 2016 / DR17

Luminosity Function method

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HO HOW ARE WE TRYING TO TO DO DO THE MEASURING? (TH THE GALA LACTI TIC / NEARBY STAR FORMING REGIONS WA WAY)

Jose et el., 2017 / Stock 8

Infrared CMD Isochrone method

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HO HOW ARE WE TRYING TO TO DO DO THE MEASURING? (TH THE GALA LACTI TIC / NEARBY STAR FORMING REGIONS WA WAY)

Suarez et el., 2019 / 25 Ori

Optical CMD Isochrone method

Log (<M>/ Msun)

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HO HOW ARE WE TRYING TO TO DO DO THE MEASURING? (TH THE GALA LACTI TIC / NEARBY STAR FORMING REGIONS WA WAY)

Muzic et el. 2019 / Rosette Nebula

Ratio of [stars : brown dwarfs]

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HO HOW ARE WE TRYING TO TO DO DO THE MEASURING? (TH THE GALA LACTI TIC / NEARBY STAR FORMING REGIONS WA WAY)

Hosek et al. 2019 / Arches

Spectroscopy method

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SP SPECTROSC SCOPY IS S RE REQUIRE RED

Photometry is cheap. Spectroscopy is still the bottleneck!

Dahm & Hillenbrand 2015

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WH WHAT ARE WE WE TRYI YING TO MEASURE?

Hopkins, 2018 Kouwenhoven et al.

dN(M)/dM α M-α dN(logM)/dlogM α MΓ

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WH WHAT IS THE BEST WAY OF COMPARING IMFS AMONG ST STAR FORMING REGIONS S AND ENVIRONMENTS? S?

  • Discrete IMF shapes è mpeak
  • Gamma-plot (or alpha-plot)
  • Ratio of stars to brown dwarfs
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dN(M)/dM α M-α dN(logM)/dlog M α MΓ Galactic clusters and OB assoc. h/χ Per; Pleiades; M35; Praesepe Stars in molecular clouds Orion Nebula Cluster NGC 3603, Wd1, Wd2, R136, etc.

CO COMPARING THE MEASURED STELLAR/ SU SUB-ST STELLAR IN INIT ITIA IAL MASS FU FUNCTION ON IN THE MILKY WAY

Γ = (1−α) after Scalo (1998), Hillenbrand (2004) Log (<M>/ Msun)

typical random error in slope

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Some claims of “top heavy” IMFs in extreme star-forming environments, but evidence is not consistent. Would a ”logistic” type function better describe the form over all masses?

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CO COMPARING THE MEASURED STELLAR INIT INITIA IAL MASS FUNC NCTIO ION N AROUND ND THE LOCAL GROUP

Weisz (2015)

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WH WHAT CAN WE WE CONCLUDE THUS FAR?

  • The IMF has a constant slope above about 0.8-1.0 Msun, consistent

with the original Salpeter power law – on average.

  • Below about 0.8 Msun, the IMF slope changes constantly with

decreasing mass, consistent with lognormal or smoothed/tapered two-power-law forms.

  • Below ~0.1 Msun a single power-law may again prevail.
  • At all masses, there is dispersion in the measured slopes at the +/- 0.5

dex level, above claimed measurement errors but perhaps not above systematic effects inherent in the different methods.

  • The fragmentation limit for star formation has not yet been measured.
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WH WHERE SHOULD WE WE DO IT? SO SOME FAVORITE REGIONS S ARE “N “NON-ID IDEAL” FO FOR JWST

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WH WHERE SHOULD WE WE DO IT? SO SOME FAVORITE REGIONS S ARE WELL-SU SUITED FOR JWST ST

Lu (2019) Hosek et al. (2019, DSWP)

  • ~2-6 Myr old
  • Each cluster has ~100-150 O-type stars and contains

total stellar mass >104 Msun.

  • Spatial resolution, sensitivity, infrared all required.

Currently sampled to only 1.8 Msun.

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WH WHERE SHOULD WE WE DO IT? SO SOME FAVORITE REGIONS S ARE WELL-SU SUITED FOR JWST ST

Andersen (2009)

  • ~3 Myr old
  • ~100-150 O-type stars and >5x104 Msun.
  • Spatial resolution and sensitivity required.

Currently sampled to only 1.1 Msun.

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graphic courtesy of M. McCaughrean

NE NEARBY CLUSTERS GTO PROGRAMS (P (PHOTOMETRY RY AND SPECTRO ROSCOPY)

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graphic courtesy of M. McCaughrean

NE NEARBY CLUSTERS GTO PROGRAMS (S (SPECTRO ROSCOPY)

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OLD OLD QUESTION ONS RE THE IN INIT ITIA IAL MASS FUNCTIO ION

  • Is the IMF the same everywhere, i.e. invariant with respect to:
  • time?
  • location?
  • size or density of stellar environment?
  • metallicity?
  • other initial cloud conditions, e.g. temperature or pressure?
  • Can we relate IMF to some physical aspect of star formation?
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Hopkins, 2018