The present-day star formation rate of the Milky-Way determined from Spitzer detected young stellar objects (1001.3672v1)

Abstract: We present initial results from a population synthesis model aimed at determining the star formation rate of the Milky-Way. We find that a total star formation rate of 0.68 to 1.45 Msun/yr is able to reproduce the observed number of young stellar objects in the Spitzer/IRAC GLIMPSE survey of the Galactic plane, assuming simple prescriptions for the 3D Galactic distributions of YSOs and interstellar dust, and using model SEDs to predict the brightness and color of the synthetic YSOs at different wavelengths. This is the first Galaxy-wide measurement derived from pre-main-sequence objects themselves, rather than global observables such as the total radio continuum, Halpha, or FIR flux. The value obtained is slightly lower than, but generally consistent with previously determined values. We will extend this method in the future to fit the brightness, color, and angular distribution of YSOs, and simultaneously make use of multiple surveys, to place constraints on the input assumptions, and reduce uncertainties in the star formation rate estimate. Ultimately, this will be one of the most accurate methods for determining the Galactic star formation rate, as it makes use of stars of all masses (limited only by sensitivity) rather than solely massive stars or indirect tracers of massive stars.

• The paper introduces a population synthesis model that directly counts pre-main-sequence YSOs, bypassing traditional tracers like Hα or radio continuum.
• It employs synthetic SEDs, an axisymmetric star formation framework, and a Kroupa IMF to simulate the brightness and distribution of YSOs in the Galaxy.
• The analysis finds an SFR range of 0.68 to 1.45 M☉/yr, aligning with previous studies while addressing uncertainties in YSO classification and observational completeness.

An Evaluation of the Present-Day Galactic Star Formation Rate Using Spitzer Detected Young Stellar Objects

The paper "The present-day star formation rate of the Milky-Way determined from Spitzer detected young stellar objects" authored by Thomas P. Robitaille and Barbara A. Whitney offers a detailed examination of the star formation rate (SFR) in the Milky Way. Utilizing data from the Spitzer/IRAC GLIMPSE survey, the researchers developed a population synthesis model to calculate the Galactic SFR by directly counting young stellar objects (YSOs), diverging from conventional methods that rely on indirect tracers.

Methodological Framework

This paper represents the first comprehensive attempt to measure the SFR of the entire Galaxy using pre-main-sequence objects rather than global observables like radio continuum, Hα, or FIR flux. The research employed a sophisticated population synthesis model, integrating the distribution of YSOs and interstellar dust within the Galaxy, alongside synthetic spectral energy distributions (SEDs) to simulate the brightness and color of YSOs across different wavelengths.

Key assumptions underlying the model include:

• An axisymmetric distribution of star-forming regions based on known gas densities and the Schmidt-type law.
• Kroupa's initial mass function (IMF) spanning masses from 0.1 to 50 solar masses.
• The ages of YSOs are selected under the assumption of constant star formation over a few million years, providing a range of 1,000 years to 2 Myr.
• A simple double exponential model for the 3D distribution of Galactic dust.
• Use of model SEDs from prior work to calculate intrinsic magnitudes for synthetic YSOs.

Results and Analysis

The analysis determined an SFR ranging from 0.68 to 1.45 M☉/yr under a Kroupa IMF, slightly lower yet consistent with previous estimates that employed other methods. The potential variation in these findings highlights lingering uncertainties in classifying observed YSOs, completeness of observations, and assumptions regarding the YSO brightness as influenced by disk and envelope properties.

The longitude and latitude distribution, as well as the brightness and color distribution of the synthetic YSOs, were found to be in reasonable agreement with the observed YSO candidates. The observed asymmetry in the latitude distribution was interpreted as resulting from the Sun's position above the Galactic plane.

Implications and Future Directions

From a theoretical standpoint, this paper emphasizes the significance of considering YSOs of all masses rather than focusing solely on massive stars which dominate most luminosity-based metrics. The use of directly observed pre-main-sequence stars introduces a more intrinsic measure of the Galactic SFR, reducing the reliance on indirect extrapolation.

Practically, the framework set forth in this paper paves the way for more refined models that consider additional parameters like angular, brightness, and color distributions of objects from various infrared surveys. These developments will not only enhance the precision of SFR determinations but may also provide deeper insights into star formation processes at scalable Galactic levels.

Future investigations could incorporate extending the methodology to integrate data from ongoing surveys, such as those from the UKIDSS or Herschel space observatories, to refine constraints on model assumptions. Enhanced sensitivity in future surveys may reduce the need for IMF extrapolation, potentially enabling an accurate assessment of SFR across different stellar mass ranges.

Such advancements will contribute immensely to our understanding of the Milky Way's structural and evolutionary parameters, delivering higher accuracy in modeling Galactic dynamics. Overall, the novel approach presented by Robitaille and Whitney signifies an evolution in accurately estimating cosmic star formation activities, with promising prospects for comprehensively mapping stellar birth rates across the Universe.