1 of 15

The Stellar Initial Mass Function Inferred from Unresolved Young Stellar Population

Beomdu Lim

Kongju National University

@SMiLE workshop

2 of 15

Mass distribution of stars with the same origin (Salpeter 1955; Kroupa 2001)

 

Kroupa (2001)

 

Initial mass function (IMF)

A functional form:

 

=

  • const.

3 of 15

Stellar IMF

  • Understanding star formation process
  • Interpreting integrated light from

unresolved stellar population

4 of 15

Long-standing debate

Universality

Star formation takes place

in a common process

Variety

Star formation process is controlled

by environmental factors

Following questions

If the IMF is variable, then would it be measurable?

How much the IMF varies, and what physical processes are responsible (Hopkins 2018)

5 of 15

Lim et al. (2014, 2017)

IMF of the Galactic open clusters

A total of 16 clusters in the Galaxy

A systematic variation of IMFs?

6 of 15

Integrated spectroscopy

Less biased sample of young massive clusters over face-on galaxies

Spectral features sensitive to the ages and IMFs

Possible to investigate IMF variations in different environments

Bastian et al. (2009)

7 of 15

Targets in different star-forming environments

External force

Starbursts

Low-metallicity

8 of 15

Simple stellar population model STARBURST99 (Lietherer et al. 1999; 2005; 2010; 2014)

Stellar population model for spectral analysis

  • Geneva stellar evolutionary models : non-rotation and rotation(V40)
  • Metallicity : Z = 0.014 and 0.002
  • Underlying IMFs : 𝚪 = 0.0 to -2.5 with an interval of 0.1
  • Cluster age : 1 to 15 Myr with an interval of 0.5

9 of 15

Spectral features depending on age and IMFs

10 of 15

Analysis

Spectroscopic age constraints

Estimation of the initial reddening value (Na I D)

Spectral matching

Top three models

Age, reddening, mass, underlying IMF

11 of 15

Clusters in NGC 4038/9

(Koo et al. 2025, AJ, 169, 7)

A total of 12 young massive clusters

12 of 15

Clusters in NGC NGC 1313

(Koo et al. in prep.)

A total of 11 young clusters

13 of 15

Mass-Г relationship?

  • Steeper IMF in more massive clusters
  • Similar correlations in two different

mass scales

  • Possible interpretations

Intrinsic feature

Systematic errors in STARBURST99

14 of 15

Further study

  • Confirming the mass-Γ relationship in a wide range of cluster masses
  • Observational testing simple stellar population models�(Gaia spectra, 7DT … for well-studied clusters)

15 of 15

Stay turned!

Q&A