Massive Stars Near and Far Proceedings IAU Symposium No. 361 2022 N. St-Louis J. S. Vink J. Mackey eds.2022 International Astronomical Union

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Massive Stars Near and Far

Proceedings IAU Symposium No. 361, 2022

N. St-Louis, J. S. Vink & J. Mackey, eds.

DOI: 00.0000/X000000000000000X

Massive stars in metal-poor dwarf galaxies

are often extreme rotators

Abel Schootemeijer1, Danny J. Lennon2,3, Miriam Garcia4, Norbert

Langer1,5, Ben Hastings1,5, and Christoph Sch¨urmann1,5

1Argelander-Institut f¨ur Astronomie, Universit¨at Bonn, Auf dem H¨ugel 71, 53121 Bonn,

Germany

2Instituto de Astrof´ısica de Canarias, E-38200 La Laguna, Tenerife, Spain

3Departamento de Astrof´ısica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain

4Centro de Astrobiolog´ıa, CSIC-INTA. Crtra. de Torrej´on a Ajalvir km 4, E-28850 Torrej´on de

Ardoz (Madrid), Spain

5Max-Planck-Institut f¨ur Radioastronomie, Auf dem H¨ugel 69, 53121 Bonn, Germany

Abstract. We probe how common extremely rapid rotation is among massive stars in the early

universe by measuring the OBe star fraction in nearby metal-poor dwarf galaxies. We apply a

new method that uses broad-band photometry to measure the galaxy-wide OBe star fractions in

the Magellanic Clouds and three more distant, more metal-poor dwarf galaxies. We ﬁnd OBe star

fractions of ∼20% in the Large Magallanic Cloud (0.5Z), and ∼30% in the Small Magellanic

Cloud (0.2Z) as well as in the so-far unexplored metallicity range 0.1.Z/Z<0.2 occupied

by the other three dwarf galaxies. Our results imply that extremely rapid rotation is common

among massive stars in metal-poor environments such as the early universe.

1. Introduction

Massive stars in the early universe are thought to be metal-poor because after the

Big Bang, the universe consisted almost exclusively of hydrogen and helium. Nearby

dwarf galaxies are unique laboratories to study the early universe because they are both

metal poor (Mateo 1998) and nearby enough to resolve individual massive stars. The

evolution of these massive stars at low metallicity is thought to be strongly impacted

by extremely rapid rotation (e.g., Langer 1992). In line with this is that superluminous

supernovae, long-duration gamma-ray bursts, and ultra-luminous X-ray sources (all of

which have been linked to rapid rotation - e.g., Aguilera-Dena et al. 2018; Marchant et al.

2016) are observed to occur mainly in low-metallicity dwarf galaxies (Lunnan et al. 2014;

Kaaret et al. 2017). Diagnostics for extremely rapid rotation in stars are the presence of

Hαemission and excess ﬂux at long wavelengths, typically thought to originate from a

decretion disk (see, e.g., Struve 1931; Poeckert & Marlborough 1976; Vink et al. 2009).

Stars that display these features are classiﬁed as OBe stars. Here we investigate the

prevalence of such rapidly rotating OBe stars in nearby metal-poor dwarf galaxies.

2. Methods

We employ archival broad-band photometry to separate OBe stars, main sequence

(MS) stars without disks, and cooler helium-burning (HeB) stars. We use colors at short

wavelengths to identify hot stars and colors at longer wavelengths to infer the presence

of a disk. Below we describe the data sets used in this work.

arXiv:2210.01453v1 [astro-ph.SR] 4 Oct 2022

2 Abel Schootemeijer et al.

Figure 1. Left: color-magnitude diagram of bright sources in the Small Magellanic Cloud.Right:

stacked color histograms showing the number of sources within the main sequence (MS, ma-

genta), OBe (orange), and helium-burning (HeB, green) groups, as identiﬁed by the color-color

cuts displayed in Fig. 2.

2.1. Magallanic Cloud data sets

For the Small and Large Magellanic Cloud (SMC and LMC), we cross-correlate†the

data from GAIA EDR3 (Gaia Collaboration et al. 2021), with classical UB photometry

(Zaritsky et al. 2002, 2004) and infrared Spitzer-SAGE photometry (Meixner et al. 2006),

adopting a 1” radius.

2.2. Sextans A, Holmberg I, and Holmberg II data sets

Sextans A, Holmberg I, and Holmberg II are about twenty to a hundred times further

away than the Magellanic Clouds. For that reason, only data from the Hubble Space

Telescope (HST) is deep enough for our method while at the same time having a high

enough angular resolution. For Sextans A, we cross-correlate‡F336Wand F439Wdata

from Bianchi et al. (2012) with F555Wand F814Wdata from Holtzman et al. (2006).

Before cross-correlating, we shift the data from Bianchi et al. (2012) by 0.6” in right

ascension and 1” in declination. The cross-correlation radius is 0.35”. For Holmberg I

and Holmberg II we use the photometric data from the Legacy ExtraGalactic UV Survey

(LEGUS; Sabbi et al. 2018).

3. Results

3.1. The Magellanic Clouds

The color-magnitude diagram (CMD) of the SMC (Fig. 1) shows several features. Here we

focus on the two bluest bands, at Gbp −Grp ≈ −0.25 and Gbp −Grp ≈0. To investigate

the nature of the sources in these bands, we plot the sources shown in our CMD also in a

color-color diagram (Fig. 2, left). In this diagram, three groups form. At the right side of

†Using http://cdsxmatch.u-strasbg.fr/

‡Using the Aladin sky atlas

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MassiveStarsNearandFarProceedingsIAUSymposiumNo.361,2022N.St-Louis,J.S.Vink&J.Mackey,eds.©2022InternationalAstronomicalUnionDOI:00.0000/X000000000000000XMassivestarsinmetal-poordwarfgalaxiesareoftenextremerotatorsAbelSchootemeijer1,DannyJ.Lennon2;3,MiriamGarcia4,NorbertLanger1;5,BenHastings1;5,andCh...

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Massive Stars Near and Far Proceedings IAU Symposium No. 361 2022 N. St-Louis J. S. Vink J. Mackey eds.2022 International Astronomical Union

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