A Search for In-Situ Field OB Star Formation in the Small Magellanic Cloud

Abstract

Whether any OB stars form in isolation is a question central to theories of massive star formation. To address this, we search for tiny, sparse clusters around 210 field OB stars from the Runaways and Isolated O-Type Star Spectroscopic Survey of the SMC (RIOTS4), using friends-of-friends (FOF) and nearest neighbors (NN) algorithms. We also stack the target fields to evaluate the presence of an aggregate density enhancement. Using several statistical tests, we compare these observations with three random-field datasets, and we also compare the known runaways to non-runaways. We find that the local environments of non-runaways show higher aggregate central densities than for runaways, implying the presence of some "tips-of-iceberg" (TIB) clusters. We find that the frequency of these tiny clusters is low, $\sim 4-5\%$ of our sample. This fraction is much lower than some previous estimates, but is consistent with field OB stars being almost entirely runaway and walkaway stars. The lack of TIB clusters implies that such objects either evaporate on short timescales, or do not form, implying a higher cluster lower-mass limit and consistent with a relationship between maximum stellar mass ($m_{\max }$) and the mass of the cluster ($M_{\mathrm{cl}}$). On the other hand, we also cannot rule out that some OB stars may form in highly isolated conditions. Our results set strong constraints on the formation of massive stars in relative isolation.

24 pages, 13 figures, Accepted to ApJ

References in corpus (8)

• The High Angular Resolution Multiplicity of Massive Stars
• Gas expulsion and the destruction of massive young clusters
• A Minimum Column Density of 1 g cm^-2 for Massive Star Formation
• Do O-stars form in isolation?
• Star cluster "infant mortality" in the Small Magellanic Cloud (Redivivus)
• Identifying star clusters in a field: A comparison of different algorithms
• Runaway OB Stars in the Small Magellanic Cloud: Dynamical Versus Supernova Ejections
• A MST algorithm for source detection in gamma-ray images

Cited by in corpus (8)

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• Runaway OB Stars in the Small Magellanic Cloud: Dynamical Versus Supernova Ejections
• A Panchromatic Study of Massive Stars in the Extremely Metal-Poor Local Group Dwarf Galaxy Leo A
• Strong Variability in AzV 493, an Extreme Oe-Type Star in the SMC
• New Field OB and OBe Binaries of the SMC Wing: Observational Properties and Population Modeling
• Sapaki: Galactic O3If* star possibly born in isolation
• Isolated massive star candidates in NGC 4242 with GULP