Near-infrared spectroscopic observations of massive young stellar object candidates in the central molecular zone

G Nandakumar,A Rojas-Arriagada,R Schödel,A Tej,F Matteucci,A Feldmeier-Krause,M Schultheis,N Ryde,N Neumayer

doi:10.1051/0004-6361/201731918

Abstract

Context. The central molecular zone (CMZ) is a ~200 pc region around the Galactic centre. The study of star formation in the central part of the Milky Way is of great interest as it provides a template for the closest galactic nuclei. Aims. We present a spectroscopic follow-up of photometrically selected young stellar object (YSO) candidates in the CMZ of the Galactic centre. Our goal is to quantify the contamination of this YSO sample by reddened giant stars with circumstellar envelopes and to determine the star formation rate (SFR) in the CMZ. Methods. We obtained KMOS low-resolution near-infrared spectra (R ~ 4000) between 2.0 and 2.5 μm of sources, many of which have been previously identified by mid-infrared photometric criteria as massive YSOs in the Galactic centre. Our final sample consists of 91 stars with good signal-to-noise ratio. We separated YSOs from cool late-type stars based on spectral features of CO and Brγ at 2.3 μm and 2.16 μm, respectively. We made use of spectral energy distribution (SED) model fits to the observed photometric data points from 1.25 to 24 μm to estimate approximate masses for the YSOs. Results. Using the spectroscopically identified YSOs in our sample, we confirm that existing colour–colour diagrams and colour-magnitude diagrams are unable to efficiently separate YSOs and cool late-type stars. In addition, we define a new colour–colour criterion that separates YSOs from cool late-type stars in the H−KS vs. H −[8.0] diagram. We use this new criterion to identify YSO candidates in the |l| < 1.̊5, |b| < 0.̊5, region and use model SED fits to estimate their approximate masses. By assuming an appropriate initial mass function (IMF) and extrapolating the stellar IMF down to lower masses, we determine a SFR of ~0.046 ± 0.026 M⊙ yr-1 assuming an average age of 0.75 ± 0.25 Myr for the YSOs. This value is lower than estimates found using the YSO counting method in the literature. Conclusions. Our SFR estimate in the CMZ agrees with the previous estimates from various methods and reaffirms that star formation in the CMZ is proceeding at a lower rate than predicted by various star forming models.

Highlights

The central molecular zone (CMZ) is the innermost ∼200 pc region of the Milky Way, covering about −0◦.7 < l < 1◦.8 in longitude and −0◦.3 < b < 0◦.2 in latitude
With the aim of estimating the star formation rate (SFR) in the CMZ using spectroscopic identification of young stellar object (YSO), we prepared a detailed observation of 22 fields using Kband Multi Object Spectrograph (KMOS)
Based on the CO absorption found in cool, late-type stars and Brγ emission seen in YSOs, we were able to clearly separate YSOs from cool, late-type stars in the equivalent width (EW)(CO) vs. EW(Brγ) diagram

Summary

Introduction

The central molecular zone (CMZ) is the innermost ∼200 pc region of the Milky Way, covering about −0◦.7 < l < 1◦.8 in longitude and −0◦.3 < b < 0◦.2 in latitude. It is a giant molecular cloud complex with an asymmetric distribution of molecular clouds The gas pressure and temperature are higher in the CMZ than in the Galactic disk; these conditions favour a larger Jeans mass for star formation and an initial mass function (IMF) biased towards more massive stars (see Serabyn & Morris 1996; Fatuzzo & Melia 2009). It is essential to understand the modes of star formation and star formation history in the unique environment of the CMZ, both to gain insight into our own Milky Way and to provide a template for circumnuclear star formation in the closest galactic nuclei

Objectives

Methods

Findings

Conclusion