Astronomy & Astrophysics manuscript no. main ©ESO 2021 January 14, 2021 Radio observations of massive stars in the Galactic centre: The Arches Cluster Gallego-Calvente1,?, A. T., Schödel1, R., Alberdi1, A., Herrero-Illana2, R., Najarro3, F., Yusef-Zadeh4, F., Dong, H., Sanchez-Bermudez5; 6, J., Shahzamanian1, B., Nogueras-Lara6, F., and Gallego-Cano7, E. 1 Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain e-mail: [email protected] 2 European Southern Observatory (ESO), Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile 3 Centro de Astrobiología (CSIC/INTA), Ctra. de Ajalvir Km. 4, 28850 Torrejón de Ardoz, Madrid, Spain 4 CIERA, Department of Physics and Astronomy Northwestern University, Evanston, IL 60208, USA 5 Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 70264, Ciudad de México 04510, México 6 Max-Planck-Institut für Astronomie, Königstuhl 17, Heidelberg, D-69 117, Germany 7 Centro Astronómico Hispano-Alemán (CSIC-MPG), Observatorio Astronómico de Calar Alto, Sierra de los Filabres, 04550, Gér- gal, Almería, Spain ABSTRACT We present high-angular-resolution radio observations of the Arches cluster in the Galactic centre, one of the most massive young clusters in the Milky Way. The data were acquired in two epochs and at 6 and 10 GHz with the Karl G. Jansky Very Large Array (JVLA). The rms noise reached is three to four times better than during previous observations and we have almost doubled the number of known radio stars in the cluster. Nine of them have spectral indices consistent with thermal emission from ionised stellar winds, one is a confirmed colliding wind binary (CWB), and two sources are ambiguous cases. Regarding variability, the radio emission appears to be stable on timescales of a few to ten years. Finally, we show that the number of radio stars can be used as a tool for constraining the age and/or mass of a cluster and also its mass function. 1. Introduction tral index at certain orbital phases in systems with significant ec- centricity. On the other hand, theoretical studies suggest that the Massive stars are of fundamental importance to understanding free-free thermal emission from the WCR may also affect the to- star formation and galaxy evolution because of their key role in tal radio spectrum, a phenomenon that becomes more important stirring and enriching the interstellar medium through intense as the stars get very close, increasing the spectral index to values ionising radiation, stellar winds, and supernovae. A striking fea- steeper than the nominal ionised wind at millimeter wavelengths ture of massive stars is their high fraction of multiplicity (∼ 91 %, (Montes et al. 2015). Sana et al. 2014), which must therefore be taken into account Past radio observations of massive stars lacked the sensitiv- when studying their evolution and the heavy end of the initial ity to reach beyond at most a few kiloparsecs. Because of the mass function. Considering that the wind properties of massive rareness and large mean distances of massive stars and clusters, stars are still poorly observationally constrained (see Benaglia researchers could were therefore only able to study a limited 2010), radio observations can provide further information about sample of such targets (Lang et al. 2001, 2005; Benaglia 2010; the wind properties and multiplicity of the most massive stars. Yusef-Zadeh et al. 2015). As concerns the properties of radio stars, for isolated stars The Galactic centre (GC) region occupies less than 1% of α we expect a spectral index of α ≈ 0:6 (flux density S ν / ν ) the volume of the Milky Way disc, but emits on the order 10% arising from the thermal emission of the ionised wind, while bi- of its total Lyman continuum flux, which is produced by a high naries can show flat to inverted spectra (α . 0) which result from number of massive, hot stars (e.g. Figer et al. 2004; Mauerhan the contribution of non-thermal emission in colliding wind re- et al. 2010; Dong et al. 2011). The Arches, Quintuplet, and the gions, provided this emission is not absorbed by the surrounding Central Parsec massive young clusters lie within 30 pc in projec- ionised wind, which is optically thick (Benaglia 2010; Montes tion of the central black hole Sagittarius A* (Sgr A*) and each 4 et al. 2015). Therefore, identifying binaries through radio obser- contain & 10 M of stars that formed 2-6 Myr ago (Figer et al. vations is typically limited to the detection of wide binaries with 2004). Massive young stellar objects (YSOs) and H II regions arXiv:2101.05048v1 [astro-ph.SR] 13 Jan 2021 periods of one year or longer (Sanchez-Bermudez et al. 2019). throughout the GC are further witnesses to recent or currently In the case of short-period (P ∼< 1 yr) colliding wind binaries ongoing massive star formation (SF) throughout the GC (e.g. (CWBs), the stars are too close and the wind–wind collision re- Yusef-Zadeh et al. 2009; Mauerhan et al. 2010; Nandakumar gion (WCR) is likely to lie within the optically thick region of et al. 2018; Shahzamanian et al. 2019). In summary, the GC is the winds. Therefore, in these systems, only the free-free thermal the Milky Way’s most important star forming region. The condi- emission from the unshocked winds is thought to be detected, tions in the GC resemble those in high-redshift starburst galaxies thereby masking any effect of their binarity. However, for some (Kruijssen & Longmore 2013). Thus, the GC is of special impor- short-period systems, the non-thermal emission escapes the ab- tance for studies of massive star formation. sorption, contributing to a composite spectrum with a flat spec- However, there are some unique observational challenges for studying the stellar population of the GC. Interstellar extinction ? e-mail: [email protected] is extremely high (AV > 30 mag) and also variable on arcsec- Article number, page 1 of 11 A&A proofs: manuscript no. main Table 1. Observational properties 1 h m s servatory (NRAO) . The position α, δ(J2000) = 17 45 50.49 , ◦ 0 00 a −28 49 19.92 was taken as phase centre. There are three Observation Band JVLA On source time epochs of X-band (3.0 cm or a representative frequency of date configuration (minutes) 10 GHz) and one of C-band (5.0 cm or 6 GHz) observations. De- Oct 04, 2016 X A 55 tails are listed in Table 1. All the observations in all epochs were Oct 26, 2016 X A 55 taken in the A configuration to achieve the highest angular reso- Apr 11, 2018 X A 55 lution. This configuration also helped us to filter out part of the Jun 10, 2018 C A 74 extended emission from the Arched Filaments (G0.10+0.08), a very extended H II region in which the Arches cluster is embed- (a) Notes. Frequency range of 4-8 GHz for C-band and 8-12 GHz for ded. X-band. Therefore, the total bandwidth was 4 GHz on each band. The At all frequencies, J1744−3116 was used as a phase cal- number of spectral windows was 32 and the number of channels 64 in both cases. ibrator and J1331+305 (3C286) as a band-pass and flux den- sity calibrator. For C- and X-bands, we required a sensitivity of 3.0 µJy beam−1. Raw data were processed automatically through ond scales (e.g. Scoville et al. 2003; Schödel et al. 2010; Fritz the JVLA calibration pipeline performing an initial flagging and et al. 2011; Nogueras-Lara et al. 2018, 2019b, 2020). There- calibration. Extra flagging was necessary to remove the lost or fore, even in the near-infrared (AK ≈ 2:5 mag), stellar colours corrupted data. We performed standard data reduction using the are dominated by reddening. Finally, sub-arcsecond angular res- Common Astronomy Software Applications package (CASA) olution (at least as good as about 0.200) is needed to overcome developed by an international consortium of scientists2. the high source crowding and to reliably study individual stars, Images were created with CASA using the classical task which requires the use of the Hubble Space Telescope, or speckle clean in interactive mode. We also tested a more advanced form or adaptive optics techniques from the ground. of imaging, multi-scale clean, to distinguish between the point In this work, we focus on the Arches cluster that is located sources and the extended emission. multi-scale clean involves at a projected distance of ∼ 26 pc to the northeast of Sgr A* (110, the use of multiple scales by means of an extension of the classi- 4 angular distance). It contains a few × 10 M with more than 100 cal clean algorithm assuming the sky is composed of emission at O-stars and thus belongs to a small handful of young, massive different angular scales. Its use did not improve the quality of the starburst-like clusters known in the Milky Way (in addition to, image. We also probed various other options for the task clean e.g. NGC3603, Quintuplet, or Westerlund 1 and 2). The cluster including wide-field, multi-term, multi-frequency synthesis and was formed between 2 and 4 Myr ago (e.g. Figer et al. 1999a,b; with w-projection without resulting in any significant improve- Najarro et al. 2004; Clarkson et al. 2012; Clark et al. 2018, 2019) ments. Additionally, we checked different weighting schemes to and has a half-light radius of rh ≈ 0:48 pc (Hosek et al. 2015). correct for visibility sampling effects. Natural weighting pro- Clark et al.