[Paper Review] The VLT-FLAMES Tarantula Survey XIV. The O-Type Stellar Content of 30 Doradus

N. R. Walborn, H. Sana+7 more|UvA-DARE (University of Amsterdam)|Feb 27, 2014

Stellar, planetary, and galactic studiesPhysics and Astronomy66 references60 citations

TL;DR

This paper presents high-resolution spectral classifications of 352 O- and B0-type stars in the 30 Doradus region from the VLT-FLAMES Tarantula Survey, identifying 213 high-quality O-type stars. Key findings include the discovery of 18 extremely rapidly rotating O-type stars (with $v\sin i \sim 600$ km s$^{-1}$), a large population of young Vz stars, and a significant fraction of O V((fc)) stars with strong C III emission, advancing understanding of massive star evolution and cluster dynamics in the LMC's starburst environment.

ABSTRACT

Detailed spectral classifications are presented for 352 O-B0 stars in the VLT-FLAMES Tarantula Survey, of which 213 O-type are of sufficient quality for further morphological analysis. Among them, six subcategories of special interest are distinguished. (1) Several new examples of the earliest spectral types O2-O3 have been found. (2) A group of extremely rapidly rotating main-sequence objects has been isolated, including the largest $v\sin i$ values known, the spatial and radial-velocity distributions of which suggest ejection from the two principal ionizing clusters. (3) Several new examples of the evolved, rapidly rotating Onfp class show similar evidence. (4) No fewer than 48 members of the Vz category, hypothesized to be on or near the ZAMS, are found in this sample; in contrast to the rapid rotators, they are strongly concentrated to the ionizing clusters, supporting their interpretation as very young objects, as do their relatively faint absolute magnitudes. (5) A surprisingly large fraction of the main-sequence spectra belong to the recently recognized V((fc)) class, with C III emission lines of similar strength to the usual N III in V((f)) spectra; there are also six objects with very high-quality data but no trace of either mission feature, presenting new challenges to physical interpretations. (6) Five spectra with morphologically enhanced nitrogen lines have been detected. Absolute visual magnitudes have been derived for each star with individual extinction laws, and composite HRDs provide evidence of the multiple generations present in this field. Associations with X-ray sources are noted. Further analyses of this unique dataset underway will provide new insights into the evolution of massive stars and starburst clusters.

Motivation & Objective

• To provide detailed spectral classifications for O- and B0-type stars in the 30 Doradus starburst region using high-resolution VLT-FLAMES data.
• To identify and characterize rare or unusual O-type stellar subtypes, including extreme rotators and evolved rapidly rotating stars.
• To investigate the physical significance of morphological features such as Vz, V((fc)), and enhanced nitrogen lines in the context of low-metallicity massive star evolution.
• To support broader astrophysical analyses by providing a foundation of accurate spectral types and extinction-corrected absolute magnitudes for the VFTS dataset.

Proposed method

• High-resolution spectroscopy ($R \sim 10^4$) was used to classify 352 O- and B0-type stars from the VLT-FLAMES Tarantula Survey.
• A high-quality subset of 213 O-type stars was selected based on signal-to-noise ratio, resolution, and multiplicity status to ensure reliable morphological analysis.
• Spectral classification was performed using a new atlas of Galactic O-star spectra, with reference to established systems like GOSSS and IACOB.
• Extinction corrections were applied individually to each star using tailored extinction laws to derive accurate absolute visual magnitudes.
• Spatial and radial velocity distributions were analyzed to investigate the dynamical origin of extreme rotators.
• Associations with Chandra X-ray sources and visual multiplicity were cross-identified using archival data and HST/WFC3 imaging.

Experimental results

Research questions

• RQ1 What is the distribution and nature of the most rapidly rotating O-type stars in 30 Doradus, and what is their dynamical origin?
• RQ2 What physical significance underlies the Vz and V((fc)) spectral morphologies observed in O-type stars in the LMC?
• RQ3 How do the spatial and luminosity distributions of Vz stars support their interpretation as very young, pre-main-sequence or early main-sequence objects?
• RQ4 What fraction of O-type stars in 30 Doradus exhibit enhanced nitrogen lines, and what does this imply for mass transfer or internal mixing?
• RQ5 How do the observed spectral features, including strong C III emission in O V((fc)) stars, challenge or refine current models of O-star atmospheres and winds?

Key findings

• 18 extremely rapidly rotating O-type main-sequence stars were identified, including the fastest known with $v\sin i \sim 600$ km s$^{-1}$, whose spatial and radial velocity distributions suggest ejection from the NGC 2070 and NGC 2060 clusters.
• 48 Vz stars were found, strongly concentrated near the ionizing clusters and a newly identified star-forming region to the north, supporting their interpretation as very young, pre-main-sequence or early main-sequence objects.
• 19 stars were classified as O V((fc)) due to C III emission lines of equal or greater strength than the usual N III, indicating complex atmospheric or wind conditions.
• Six high-signal-to-noise stars showed no trace of either N III or C III emission features, presenting a new challenge for spectral classification and physical modeling.
• Six new O2–O3 stars were discovered, and one previously known O-star was reclassified as nitrogen-rich ON2, expanding the known population of the earliest O-type stars.
• The study confirms a high spectroscopic binary fraction of at least 50% among single O stars, with several evolved Onfp stars showing evidence of binarity.