Norberto Castro

The extreme Oe star AzV 493 is known to show unusual photometric and spectroscopic variability that suggest the presence of an unseen companion in a highly eccentric and long-period (7.3 or 14.6 yr) orbit. We obtained a Chandra/ACIS observation near the putative periastron for the 7.3 yr orbit to test for transient X-ray emission that would confirm its binary nature. Our data only place an upper limit to the X-ray luminosity of L _X < 2.5 × 10 ³³ erg s ⁻¹ based on the 0.5–8 keV flux limit. Additionally, we obtained 4 new spectroscopic observations with the M2FS spectrograph at Magellan and 20 archive FLAMES/GIRAFFE and X-Shooter spectra from ESO/VLT to further constrain the possibility of radial velocity (RV) variation. Statistical analysis of the RV measurements yields inconclusive results regarding the existence of variations. We discuss possible mass limits for a potential companion, which may be a black hole, in the event that the variations are real. The violet-to-red Balmer ratio has also recently inverted, which may be a further indication of a companion.

Context. Bow shocks around massive stars have primarily been detected in IR emission, but radio detections are becoming more frequent with the commissioning of sensitive and large field-of-view interferometers. Radio data are used to probe both thermal and non-thermal emission in order to constrain the relativistic electron population and potentially particle-acceleration processes. Aims. We undertook a radio survey for bow shocks accessible from the Northern Hemisphere based on IR catalogues of candidates using the Very Large Array (VLA) and the 100 m Effelsberg Telescope. Our aim was to yield new detections and better characterise the multi-wavelength emission. Methods. We used Gaia DR3 to re-calculate spatial motion of the driving stars with respect to the surrounding stellar population. We studied the radio emission from bow shocks using emission maps and spectral-index measurements and compared our results with data from catalogues and multi-wavelength emission. Results. Of the 24 targets observed with the VLA in the 4–12 GHz band, six were clearly detected (including two previously reported) and five were possibly detected. A subset of these were also observed and detected with Effelsberg at 4–8 GHz. The VLA-derived spectral index maps indicate non-thermal emission for most sources, but the statistical uncertainties are large for most sources and all Effelsberg observations indicate thermal emission. Assuming thermal emission, we obtained upper limits on the electron density within the shocked layer. We also obtained upper limits on radio emission from the bow shock of ζ Oph at a similar flux level as predictions from magnetohydrodynamic simulations. Conclusions. Our survey marks a significant addition to the approximately ten previously known radio-emitting bow shocks in the literature and demonstrates that deep targeted radio surveys can have a good success rate in detecting IR-selected bow shocks. Followup observations of these targets at lower and higher frequencies are encouraged to determine whether any are non-thermal emitters such as the bow shocks of BD+43 ^° 3654, BD+60 ^° 2522, and LS 2355.

We present a radial velocity (RV) survey of the field OB and OBe stars of the SMC Wing. We use multiepoch observations of 55 targets obtained with the Magellan Inamori-Magellan Aerial Camera and Spectrograph and M2FS multi-object spectrographs to identify single- and double-lined spectroscopic binaries. We also use TESS light curves to identify new eclipsing binary candidates. We find that 10 each of our 34 OB (29%) and 21 OBe (48%) stars are confirmed binaries, and at least ∼6 more are candidates. Using our RV measurements, we set constraints on the companion masses, and in some cases, on periods, eccentricities, and inclinations. The RV data suggest that OB binaries favor more circular orbits (mean eccentricity 〈e〉 = 0.08 ± 0.02), while OBe binaries are eccentric (〈e〉 = 0.45 ± 0.04). We identify two candidate black hole binaries, [M2002] 77616, and 81941. We use Binary Population and Spectral Synthesis to predict the frequencies of ejected OB and OBe stars and binaries, assuming OBe stars are binary mass gainers ejected by the companion supernova. We also predict the frequencies of black-hole, neutron-star, and stripped-star companions, and we model the distributions of primary and secondary masses, periods, eccentricities, and velocity distributions. The models are broadly consistent with the binary origin scenario for OBe stars, and predict an even larger number of post-supernova OB binaries. Comparison with the kinematics supports a significant contribution from dynamical ejections for both OB and OBe stars, although less so for binaries.

Melnick 34 (Mk 34) is one of the most massive binary systems known and is one of the brightest X-ray point sources in the 30 Doradus region. We investigated the impact of this massive system on the surrounding interstellar medium (ISM) using the optical spectroscopic capabilities of the narrow-field mode (NFM) of the Multi-Unit Spectroscopic Explorer (MUSE). MUSE-NFM spatially resolved the ISM in the vicinity of Mk34 with a resolution comparable to that of the HST. The analysis of the [NII] λ 6583 and [SII] λ 6717 emission lines reveals a cone-like structure apparently originating from Mk 34 and extending southeast. Electron density maps and radial velocity measurements of the ISM lines further support an outflow scenario traced by these emissions. While no clear northwestern counterpart to this outflow was observed, we note increased extinction in that direction, towards the R136 cluster. The ISM material along the projected diagonal of the outflow on both sides of Mk 34 shows similar properties in terms of the emission line ratios seen in the Baldwin-Phillips-Terlevich diagram. These results are consistent across two observational epochs. Additionally, we examined the residual maps within a 0.5″ radius of Mk 34 after modeling and subtracting the point spread function. The observed variations in the residuals could potentially be linked to Mk 34’s known periodic behavior. However, further observations with appropriate cadence are needed to fully monitor the 155 day periodicity of Mk 34’s X-ray emissions.

Melnick 34 (Mk 34) is one of the most massive binary systems known and is one of the brightest X-ray point sources in the 30 Doradus region. We investigated the impact of this massive system on the surrounding interstellar medium (ISM) using the optical spectroscopic capabilities of the narrow-field mode (NFM) of the Multi-Unit Spectroscopic Explorer (MUSE). MUSE-NFM spatially resolved the ISM in the vicinity of Mk 34 with a resolution comparable to that of the HST. The analysis of the [NII]$λ$6583 and [SII]$λ$6717 emission lines reveals a cone-like structure apparently originating from Mk 34 and extending southeast. Electron density maps and radial velocity measurements of the ISM lines further support an outflow scenario traced by these emissions. While no clear northwestern counterpart to this outflow was observed, we note increased extinction in that direction, towards the R136 cluster. The ISM material along the projected diagonal of the outflow on both sides of Mk 34 shows similar properties in terms of the emission line ratios seen in the Baldwin-Phillips-Terlevich diagram. These results are consistent across two observational epochs. Additionally, we examined the residual maps within a 0.5" radius of Mk 34 after modeling and subtracting the point spread function. The observed variations in the residuals could potentially be linked to Mk 34's known periodic behavior. However, further observations with appropriate cadence are needed to fully monitor the 155 day periodicity of Mk 34's X-ray emissions.

The formation of massive O-type stars cannot be simply explained as a scaled-up version of the accretion mechanisms observed in lower-mass stars. Understanding these processes necessitates systematic studies of their early stages, which are challenging to identify. Forming massive stars remain embedded in their dense nursery clouds, and IR instruments with high spatial resolution capabilities are needed to better observe them. Despite these challenges, MUSE optical observations of the massive cluster NGC 2070 successfully detected potential star-forming regions through spatially resolved electron density maps. To further explore these regions, theJames WebbSpace Telescope (JWST) utilized its NIRCam and MIRI instruments to penetrate optically obscured areas. This study examines two specific regions in the southeast part of the NGC 2070 MUSE density map, where tracks of highly dense point sources were identified. NIRCam, partially overlapped with MIRI, resolved these MUSE findings, revealing a procession of stellar point sources in the projected images. The detections are associated with elongated clouds, suggesting greater proper motions compared to the surrounding interstellar medium. These findings may indicate the presence of runaway candidates in the early stages of their evolution that are following common escape routes. This would support the notion that dynamical ejection is an efficient mechanism for the formation of massive runaway stars during early stages and likely plays a significant role in the origin of O-type field stars. However, additional data are required to confirm this scenario and rule out other ionizing feedback mechanisms, such as those observed in the formation of pillar-like structures around HII regions in the Milky Way. MUSE electron density mapping effectively captures the complexity of NGC 2070’s interstellar medium and highlights targets for subsequent spectroscopic follow-ups, as demonstrated by the JWST data in the two fields studied.

The formation of massive O-type stars cannot be simply explained as a scaled-up version of the accretion mechanisms observed in lower-mass stars. Understanding these processes necessitates systematic studies of their early stages. Forming massive stars remain embedded in their dense nursery clouds, and IR instruments with high spatial resolution capabilities are needed to better observe them. Despite these challenges, MUSE optical observations of the massive cluster NGC 2070 successfully detected potential star-forming regions through spatially resolved electron density maps. To further explore these regions, the JWST utilized its NIRCam and MIRI instruments to penetrate optically obscured areas. This study examines two specific regions in the southeast part of the NGC 2070 MUSE density map, where tracks of highly dense point sources were identified. NIRCam, partially overlapped with MIRI, resolved these MUSE findings, revealing a procession of stellar point sources in the projected images. The detections are associated with elongated clouds, suggesting greater proper motions compared to the surrounding interstellar medium. These findings may indicate the presence of runaway candidates in the early stages of their evolution that are following common escape routes. This would support the notion that dynamical ejection is an efficient mechanism for the formation of massive runaway stars during early stages and likely plays a significant role in the origin of O-type field stars. However, additional data are required to confirm this scenario and rule out other ionizing feedback mechanisms, such as those observed in the formation of pillar-like structures around HII regions in the Milky Way. MUSE electron density mapping effectively captures the complexity of NGC 2070's interstellar medium and highlights targets for subsequent spectroscopic follow-ups.

We use the kinematics of field OB stars to estimate the frequencies of runaway stars generated by the dynamical ejection scenario (DES), the binary supernova scenario (BSS), and the combined two-step mechanism. We update the proper motions for field OB and OBe stars in the Small Magellanic Cloud (SMC) using Gaia DR3. Our sample now contains 336 stars from the Runaways and Isolated O-Type Star Spectroscopic Survey of the SMC, and we update our algorithm to calculate more accurate velocities compared to those obtained previously from DR2. We find a decrease in median velocity from 39 to 29 km s⁻¹, implying that the proper motions from our previous work were systematically overestimated. We present the velocity distribution for OBe stars and quantitatively compare it to those of noncompact binaries and high-mass X-ray binaries. We confirm that OBe stars appear to be dominated by the BSS and are likely post-SN binary systems, further supporting the mass-transfer model to explain the origin of their emission-line disks. In contrast, normal OB stars may show a bimodal velocity distribution, which may be expected from different processes that occur with dynamical ejections. The kinematics of fast-rotating OB stars are similar to those of normal OB stars rather than OBe stars, suggesting that the origin of their high v r sin i is different from that of OBe stars. We update our model parameters describing the kinematic origins of the SMC field population, still confirming that for runaway stars, the DES mechanism dominates, and two-step ejections seem comparable in frequency to pure BSS ejections.