A dearth of young and bright massive stars in the Small Magellanic Cloud.
Abstract: Massive star evolution at low metallicity is closely connected to many fields in high-redshift astrophysics, but poorly understood. The Small Magellanic Cloud (SMC) is a unique laboratory to study this because of its metallicity of 0.2 Zsol, its proximity, and because it is currently forming stars. We used a spectral type catalog in combination with GAIA magnitudes to calculate temperatures and luminosities of bright SMC stars. By comparing these with literature studies, we tested the validity of our method, and using GAIA data, we estimated the completeness of stars in the catalog as a function of luminosity. This allowed us to obtain a nearly complete view of the most luminous stars in the SMC. When then compared with stellar evolution predictions. We also calculated the extinction distribution, the ionizing photon production rate, and the star formation rate. Our results imply that the SMS hosts only 30 very luminous main-sequence stars (M > 40 Msol; L > 10^5 Lsol), which are far fewer than expected from the number of stars in the luminosity range 3*10^4 20 Msol, stars in the first half of their hydrogen-burning phase are almost absent. This mirrors a qualitatively similar peculiarity that is known for the Milky Way and Large Magellanic Cloud. This amounts to a lack of hydrogen-burning counterparts of helium-burning stars, which is more pronounced for higher luminosities. We argue that a declining star formation rate or a steep initial mass function are unlikely to be the sole explanations for the dearth of young bright stars. Instead, many of these stars might be embedded in their birth clouds, although observational evidence for this is weak. We discuss implications for cosmic reionization and the top end of the initial mass function.
Summary (6 min read)
- To achieve their goal of providing a more complete picture of luminous SMC stars, the authors employ three data sets in this study.
- The authors then cross-correlate it with the GAIA DR2 catalog.
- The second data set is retrieved from GAIA DR2 alone .
- Therefore the authors can use this data set to estimate the completeness of the B10 catalog.
- The authors aim is to also calculate temperatures and luminosities for stars in the B10 data set, which contains many more stars than the VSS sample.
2.1. Deriving effective temperature and luminosity
- The authors used the existing studies from the VSS sample of SMC stars , which provide spectral types as well as temperatures, to derive empirical relations of spectral types and effective temperatures (Teff).
- For stars of A-type and later, the authors used the SMC spectral type - temperature relations of Evans & Howarth (2003) and Tabernero et al. (2018).
- The authors applied the spectral type - Teff relations to infer effective temperatures for the remaining 5155 sources.
- The authors find that the predicted and observed colors match best for a reddening of 1 http://www.pas.rochester.edu/˜emamajek/EEM_dwarf_.
- To test this method, the authors show the luminosities of the sources brighter than log(L/L ) = 4.5 in Fig. 2 for the sources that are included in the VSS sample and in the B10 data set.
2.2. Investigating the completeness with GAIA photometry
- It still does not contain all of the brightest stars in the SMC.
- The trends in this second test are very similar to those described in this section.
- Both of their completeness tests imply a higher completeness than the completeness quoted for O stars in B10 itself (∼4%).
- There the authors discuss the luminosity distribution of stars in the B10 sample and compare it with theoretical predictions.
3. General population properties
- Fig. 5 shows the distribution of the B10 sources in the HRD.
- In Fig. B.5 the authors compare the HRD positions of stars in Fig. 5 to their HRD positions according to the VSS sample (if they are in the VSS sample).
- The shape of the population also remains almost intact when different spectral type - temperature relations are used, as the authors show in Fig. B.7.
- The same is true when the authors take a different input catalog (Fig. B.8, where they use Simbad instead of B10).
- This demonstrates that the results the authors present later on are robust against the choice of assumptions described in Sect.
- In order to do so, the authors employed the effective temperature and resulting expected intrinsic color (GBP −GRP)int, which they obtained as described in Sect. 2.1. We then obtained the reddening as E(GBP−GRP) = (GBP−GRP)obs− (GBP−GRP)int, where (GBP−GRP)obs is the observed GAIA color.the authors.
- The brightest stars also tend to have a slightly higher extinction, although only modestly so.
3.2. Ionizing radiation and its escape fraction
- Potsdam Wolf-Rayet (POWR) stellar atmosphere models (Hamann et al. 2015; Hainich et al. 2019) provide predictions for the ionizing photon production rate (Q) of bright SMC stars.
- This is under the rough assumption that all the H I ionizing photons are absorbed by neutral hydrogen.
- This number would decrease when the extinction continued to decrease with λ below 116 nm.
- Combining the above, it seems most probable that fesc in the SMC is far lower than their upper limit of 0.28.
3.3. Star formation rate
- Using a Kroupa (2001) IMF to extrapolate toward lower mass stars, the authors calculated the SFR of the SMC.
- To do this, the authors counted the number of stars above the 18 M track , assuming constant star formation (CSF).
- In the best-fitting scenario (where the present-day SFR is relatively low), the SFR 7− 10 Myr ago was about three times higher than for the CSF scenario.
- Because their method is based on counting massive stars, a steeper IMF would result in a higher inferred SFR.
4.1. Blue supergiants
- Interestingly, about 200 stars in Fig. 5 reside in the region between the main sequence (MS) and the RSG branch.
- Emission features make it likely that these are late-MS stars that evolved toward critical velocity in isolation (Ekström et al.
- The reason is that very high overshooting values of αov = 0.55 are necessary for the TAMS to extend that far (Schootemeijer et al. 2019).
- It therefore appears to be unavoidable to invoke either internal mixing (Langer 1991; Stothers & Chin 1992; Schootemeijer et al. 2019) or binary interaction (Braun & Langer 1995; Justham et al. 2014) to explain their presence.
4.2. Numbers of helium-burning stars and their progenitors
- The authors counted the helium-burning stars and compared the corresponding expected number of hydrogen-burning stars (based on stellar models) with the observed number of hydrogen-burning stars.
- Corrected for a completeness of about half (Table 1), this adds up to ∼450 progenitor stars.
- This poses a modest discrepancy with the expected 650.
- The fractional core helium-burning lifetime of such stars is about 7% (Schootemeijer et al. 2019).
4.3. Age and relative age distribution
- The gray dot-dashed line in Fig. 5 denotes the location of stars that are halfway through their MS lifetime.
- Instead of ∼50% of the stars being in the first half of their MS lifetime, this number is only 7% for the B10 sample.
- More likely, the explanation for the relatively larger fraction of young stars in the VSS sample could be that this sample is biased towards hot stars simply because of a greater interest in them, and young massive stars tend to be hot.
- The authors investigate in Fig. 9 whether a decreasing SFR can explain this heavily lopsided distribution of relative ages.
- From the model population, the authors drew 106 stars with random ages between 0 and 10 Myr, and random masses between 18 and 100 M .
4.4. Luminosity distribution
- The left panel in Fig. 11 shows the luminosity distribution of the stars shown in their HRD (referred to as ‘observational’; Fig. 5).
- The authors show the observed luminosity distributions corrected for completeness in the right panel of Fig. 11.
- Then, the theoretical SFH3 distribution matches the observed distribution best.
- The uncorrected luminosity distribution is fit best for Γ ≈ −1.9.
- This means that in principle, the authors can resolve the lack of bright stars by assuming a steeper IMF.
5. Number comparisons
- In this section the authors discuss the number of massive SMC stars obtained by previous studies.
- The authors first discuss the studies of the SMC massive star population that have been performed before (Humphreys & McElroy 1984; Blaha & Humphreys 1989; Massey et al. 1995).
- When stars in WR systems are excluded and completeness is not corrected for, Fig. 5 contains 22 of such massive stars (Sect. 4.2).
- This is in line with the 0.5 dex difference between the uncorrected theoretical CSF luminosity distribution and the observed distribution without WR stars above log(L/L ) = 5.5 in the left panel of Fig. 11.
- The number of helium-burning star progenitors is also significantly lower than the authors would expect a priori.
6.1.1. Steeper initial mass function
- In principle, a steeper IMF could help to explain the lack of bright stars.
- A steeper universal IMF would have dramatic implications for the early universe because it is commonly thought that toward lower metallicity, massive stars become more common (Larson & Starrfield 1971; Schneider et al. 2018b).
- This is currently an unsatisfying explanation because it cannot resolve the apparent lack of young stars.
- An explanation that would have to be ruled out is a bias against young stars (Sect. 6.1.6): because very massive stars (which live only for a short time) tend to be young, such a bias would make the IMF appear steeper than it really is.
- Moreover, the SFR in the SMC could be low enough for stochasticity to play a role (see da Silva et al. 2012).
6.1.2. Model uncertainties
- In principle, the stellar models of Schootemeijer et al. (2019) that the authors used could overpredict stellar temperatures and therefore the inferred stellar ages.
- This is significantly larger than the shift of 0.05 dex for stars at critical rotation (Paxton et al. 2019).
- The authors conclude that rotation and envelope inflation are unlikely to affect their conclusions about the dearth of young stars.
- This is meant to explain why the distribution of their sample stars in the HRD avoids cool temperatures above the 32 M track (cf. their fig.
- The reason is that they show a significant amount of hydrogen at their surfaces, which means that if they were chemically homogeneous, they would burn hydrogen in their cores.
6.1.3. Star formation history
- The authors have shown that it can explain both the lack of young and the lack of bright stars.
- If the SFH is the explanation for the inferred lack of young stars, the SFR accordingly needs to have been quenched throughout the entire SMC at the same time.
- A dearth of young massive stars has been seen in different environments.
- It is even more unlikely that something similar happened in the two other local, but separated, star-forming environments: the LMC and the Milky Way.
6.1.4. Observational biases
- Related to the ‘embedding’ scenario described below in Sect. 6.1.6, observational biases might be at play.
- B.1 that the B10 sample is complete enough for biases not to affect their conclusions about the dearth of young and bright stars.
- Next, the authors visually inspected UV images from Cornett et al. (1997), taken with the Ultraviolet Imaging Telescope (their fig.
- Therefore the authors deem the unresolved cluster explanation to be highly unlikely, as long as the bright young stars are not embedded (Sect. 6.1.6 and 6.2).
6.1.5. Binary companions
- Because luminosity scales strongly with mass, the error will be significantly smaller for unequal-mass binaries.
- To what extent this can change the apparent spectral types is not easy to gauge from first principles.
- It is unlikely that unresolved binary companions could systematically shift the apparent spectral types of the population by that much.
- An explanation of the factor-of-a-few underabundance of WR star progenitors (Sect.4.2) and of stars with log(L/L ) > 5.5 in the luminosity distribution (Sect.4.4) would still require an extreme increase in multiplicity of stars towards the high-mass end.
6.1.6. Embedding in birth clouds
- Another possible explanation for the lack of young and bright stars could be that young stars are still embedded in the clouds from which they are born.
- Naively, the authors might expect young stars in lowmetallicity environments to be less obscured because there is less dust.
- The duration of the embedding may be more relevant than the duration of the accretion phase to explain the lack of young massive stars.
6.2. Implications of the embedding scenario
- A decrease in SFR and a prolonged embedding phase appear to be the most likely to simultaneously explain the dearth of young and bright stars.
- The authors discuss the plausibility and implications of the embedding scenario below.
- If massive stars are embedded in their birth cloud for a relatively long part of their life, there must be a large number of such objects in the SMC.
- Several dozen to several hundreds of both massive YSOs and compact H II regions are indeed observed.
- The authors tested this prediction by examining observational studies in different wavelength regimes.
6.2.1. Infrared observations
- It contains 984 ’intermediate- to high-mass’ YSO candidates.
- The authors present physical parameters of 452 of these, which they can fit well to YSO models.
- For 3 of these, their temperatures above 32 kK fall in the O-star range (Table A.1) while also luminosities above log(L/L ) = 4.5 are derived.
- From the analysis of the Sewiło et al. (2013) catalog, the authors conclude that the current observations fail to agree directly with the embedding scenario.
- This scenario would only be plausible if many other embedded hot massive stars were not in this catalog.
6.2.2. Radio observations of (ultra-)compact H II regions
- It is not known how complete this catalog is, therefore the true number could be higher.
- Moreover, compact H II regions could theoretically harbor more than one young bright star.
- Conversely, it is not evident that every compact H II region in the Wong et al. (2012) catalog contains a star that matches the criteria of their missing hot and bright stars.
- The authors conclude that observations of (ultra-)compact H II regions neither rule out nor confirm the presence of several hundred deeply embedded young and bright stars.
6.2.3. Submillimeter observations
- Star-forming regions harboring proto- and young stellar objects can also be identified by their submillimeter dust emission.
- The APEX Telescope Large Area Survey of the Galaxy has provided a map of 870 µm emission in the Galactic disk using the Atacama Pathfinder 12 m submm telescope (APEX, Schuller et al. 2009).
- This claim is supported by the fact that in the Milky way, deeply embedded O-type stars have been confirmed (e.g., Messineo et al. 2018).
- A survey of 1100 µm dust emission in the SMC with the ASTE 10- meter telescope was sensitive to condensations with molecular gas masses in excess of 104 M (Takekoshi et al. 2017), which is on the same order as the mass limit estimated from ATLASGAL.
- Regardless of the exact assumptions, APEX or any other singledish submm telescope could detect only rather large star-forming associations in the SMC.
6.2.4. Theoretical expectations
- The authors also briefly considered the embedding scenario from the theoretical side.
- With such high extinction, they would most likely not pass the selection criteria for studies at optical wavelengths aimed at massive stars on the MS; if they would, their method would underestimate their luminosity by at least two orders of magnitude, so that they would not show up in their Fig.
- The authors adopted the approximation that the stars in the first half of their MS lifetime are deeply embedded in their birth cloud (cf. Fig. 9).
- The result would be that H I ionizing photons are emitted by MS stars at a rate that is four times lower than if embedding does not play a role (half of the stars are obscured, the average unobscured star has half the H I ionizing photon emission rate; see Fig.7).
- This in turn would cause the SFR inferred from Hα emission to be underestimated.
- The authors have used literature data to obtain a nearly complete overview of the brightest stars in the SMC.
- This means that the embedding scenario is challenged as well unless more of such objects are found.
- The authors suggest to further investigate the currently unsolved issue of the dearth of young and bright stars.
- Comprehending the dearth of young and bright stars in the SMC is crucial because the SMC is at this point their main stepping stone toward a better understanding of massive star evolution in the early universe.
- The authors thank the referee Tomer Shenar for a very constructive referee report, which was highly valuable for improving the discussions in the paper.
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