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The IMF of the massive star-forming region NGC 3603 from NIR adaptive optics observations
The IMF of the massive star-forming region NGC 3603 from NIR adaptive optics observations
We study the initial mass function (IMF) of NGC 3603, one of the most massive galactic star-forming regions, to answer a fundamental question in current astrophysics - is the IMF universal, or does it vary? Using our very deep high angular resolution images obtained with the NAOS-CONICA adaptive optics system at the VLT/ESO, we have successfully revealed the low-mass stellar population in the cluster core down to about 0.4 Msun (50 % completeness limit). Based on the JHKsL' color-magnitude and color-color diagrams, we first derive an average age 0.7 Myr for the pre-main sequence stars, and an upper limit of ~2.5 Myr for the main sequence stars. We find an average foreground extinction of Av = 4.5 +- 0.5 mag, with a radial increase of Delta_Av ~ 2.0 mag towards larger radii (r < 50''). From the infrared excess emission identified in the Ks - L' vs J - H color-color diagram, we measure a disk fraction of ~25 % for stars with M > 0.9 Msun in the cluster center (r < 10''). Applying a field star rejection and correcting for incompleteness, we derive the Ks-band luminosity function (LF) for stars simultaneously detected in the JHKs-bands. The LF follows a power-law with an index of alpha ~ 0.27, and shows no turnover or truncation within the detection limit. The IMF for stars within r < 110'' is reasonably fitted by a single power-law with index Gamma ~ -0.74 in the mass range of $0.4 - 20 Msun. This is substantially flatter than the Salpeter-like IMF (Gamma = -1.35). The IMF power-law index decreases from Gamma ~ -0.31 at r < 5'' to Gamma ~ -0.86 at 30'' < r < 110''. This radial steepening of the IMF mainly occurs in the inner r < 30'' field, indicating mass segregation at the very center of the starburst cluster. Analyzing the radial mass density profile, we derive a cluster core radius of ~4''.8 (~0.14 pc), and a lower limit of ~110'' (~3.2 pc) for the cluster size. We also derive an upper limit of r ~ 1260'' (~37 pc) for the cluster size adopting an estimate of the tidal radius of the cluster. Based on the de-projected stellar density distribution, we estimate the total mass and the half-mass radius of NGC 3603 to be about 1.0 - 1.6 x 10^4 Msun and 25'' - 50'' (~0.7 - 1.5 pc), respectively. The derived core radius is > 6 x 10^4 Msun pc^-3. The estimate of the half-mass relaxation time for stars with a typical mass of 1 Msun is 10 - 40 Myr, suggesting that the intermediate- and low-mass stars have not yet been affected significantly by the dynamical relaxation in the cluster. The relaxation time for the high-mass stars is expected to be much smaller, and is comparable to the age of the cluster. We can thus not conclude if the mass segregation of the high-mass stars is primordial or caused by dynamical evolution. Our observation covers at least ~67 % of intermediate- and low-mass stars in NGC 3603, and the stars residing outside the observed field can merely steepen the IMF by Delta_Gamma < 0.16. Therefore, because of the almost constant IMF beyond a radius r > 30'', we are confident that our IMF adequately describes the whole NGC 3603 starburst cluster. We also thoroughly analyze the systematic uncertainties in our IMF determination. We conclude that the power-law index of NGC 3603 including the systematic uncertainties is Gamma = -0.74^{+0.62}_{-0.47}. Our result thus supports the hypothesis of a top-heavy IMF in starbursts, especially in combination with other studies of similar clusters such as the Arches cluster and the Galactic Center cluster.
star cluster, star formation, IMF, NGC 3603, HII region
Harayama, Yohei
2007
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Harayama, Yohei (2007): The IMF of the massive star-forming region NGC 3603 from NIR adaptive optics observations. Dissertation, LMU München: Faculty of Physics
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Abstract

We study the initial mass function (IMF) of NGC 3603, one of the most massive galactic star-forming regions, to answer a fundamental question in current astrophysics - is the IMF universal, or does it vary? Using our very deep high angular resolution images obtained with the NAOS-CONICA adaptive optics system at the VLT/ESO, we have successfully revealed the low-mass stellar population in the cluster core down to about 0.4 Msun (50 % completeness limit). Based on the JHKsL' color-magnitude and color-color diagrams, we first derive an average age 0.7 Myr for the pre-main sequence stars, and an upper limit of ~2.5 Myr for the main sequence stars. We find an average foreground extinction of Av = 4.5 +- 0.5 mag, with a radial increase of Delta_Av ~ 2.0 mag towards larger radii (r < 50''). From the infrared excess emission identified in the Ks - L' vs J - H color-color diagram, we measure a disk fraction of ~25 % for stars with M > 0.9 Msun in the cluster center (r < 10''). Applying a field star rejection and correcting for incompleteness, we derive the Ks-band luminosity function (LF) for stars simultaneously detected in the JHKs-bands. The LF follows a power-law with an index of alpha ~ 0.27, and shows no turnover or truncation within the detection limit. The IMF for stars within r < 110'' is reasonably fitted by a single power-law with index Gamma ~ -0.74 in the mass range of $0.4 - 20 Msun. This is substantially flatter than the Salpeter-like IMF (Gamma = -1.35). The IMF power-law index decreases from Gamma ~ -0.31 at r < 5'' to Gamma ~ -0.86 at 30'' < r < 110''. This radial steepening of the IMF mainly occurs in the inner r < 30'' field, indicating mass segregation at the very center of the starburst cluster. Analyzing the radial mass density profile, we derive a cluster core radius of ~4''.8 (~0.14 pc), and a lower limit of ~110'' (~3.2 pc) for the cluster size. We also derive an upper limit of r ~ 1260'' (~37 pc) for the cluster size adopting an estimate of the tidal radius of the cluster. Based on the de-projected stellar density distribution, we estimate the total mass and the half-mass radius of NGC 3603 to be about 1.0 - 1.6 x 10^4 Msun and 25'' - 50'' (~0.7 - 1.5 pc), respectively. The derived core radius is > 6 x 10^4 Msun pc^-3. The estimate of the half-mass relaxation time for stars with a typical mass of 1 Msun is 10 - 40 Myr, suggesting that the intermediate- and low-mass stars have not yet been affected significantly by the dynamical relaxation in the cluster. The relaxation time for the high-mass stars is expected to be much smaller, and is comparable to the age of the cluster. We can thus not conclude if the mass segregation of the high-mass stars is primordial or caused by dynamical evolution. Our observation covers at least ~67 % of intermediate- and low-mass stars in NGC 3603, and the stars residing outside the observed field can merely steepen the IMF by Delta_Gamma < 0.16. Therefore, because of the almost constant IMF beyond a radius r > 30'', we are confident that our IMF adequately describes the whole NGC 3603 starburst cluster. We also thoroughly analyze the systematic uncertainties in our IMF determination. We conclude that the power-law index of NGC 3603 including the systematic uncertainties is Gamma = -0.74^{+0.62}_{-0.47}. Our result thus supports the hypothesis of a top-heavy IMF in starbursts, especially in combination with other studies of similar clusters such as the Arches cluster and the Galactic Center cluster.