Context. Among the intermediate-mass magnetic chemically peculiar (MCP) stars, CU Vir is one of the most intriguing objects. Its 100% circularly polarized beams of radio emission sweep the Earth as ...the star rotates, thereby making this strongly magnetic star the prototype of a class of nondegenerate stellar radio pulsars. While CU Vir is well studied in radio, its high-energy properties are not known. Yet, X-ray emission is expected from stellar magnetospheres and confined stellar winds. Aims. Using X-ray data we aim to test CU Vir for intrinsic X-ray emission and investigate mechanisms responsible for its generation. Methods. We present X-ray observations performed with XMM-Newton and Chandra and study obtained X-ray images, light curves, and spectra. Basic X-ray properties are derived from spectral modelling and are compared with model predictions. In this context we investigate potential thermal and nonthermal X-ray emission scenarios. Results. We detect an X-ray source at the position of CU Vir. With LX ≍ 3×1028 erg s−1 it is moderately X-ray bright, but the spectrum is extremely hard compared to other Ap stars. Spectral modelling requires multi-component models with predominant hot plasma at temperatures of about TX = 25 MK or, alternatively, a nonthermal spectral component. Both types of model provide a virtually equivalent description of the X-ray spectra. The Chandra observation was performed six years later than those by XMM-Newton, yet the source has similar X-ray flux and spectrum, suggesting a steady and persistent X-ray emission. This is further confirmed by the X-ray light curves that show only mild X-ray variability. Conclusions. CU Vir is also an exceptional star at X-ray energies. To explain its full X-ray properties, a generating mechanism beyond standard explanations, like the presence of a low-mass companion or magnetically confined wind-shocks, is required. Magnetospheric activity might be present or, as proposed for fast-rotating strongly magnetic Bp stars, the X-ray emission of CU Vir is predominantly auroral in nature.
Abstract
We present new ATCA multiwavelength radio measurements (range 2.1–21.2 GHz) of the early-type magnetic star ρ Oph A, performed in 2019 March during three different observing sessions. These ...new ATCA observations evidence a clear rotational modulation of the stellar radio emission and the detection of coherent auroral radio emission from ρ Oph A at 2.1 GHz. We collected high-resolution optical spectra of ρ Oph A acquired by several instruments over a time span of about 10 yr. We also report new magnetic field measurements of ρ Oph A that, together with the radio light curves and the temporal variation of the equivalent width of the He i line (λ = 5015 Å), were used to constrain the rotation period and the stellar magnetic field geometry. The above results have been used to model the stellar radio emission, modelling that allowed us to constrain the physical condition of ρ Oph A’s magnetosphere. Past XMM–Newton measurements showed periodic X-ray pulses from ρ Oph A. We correlate the X-ray light curve with the magnetic field geometry of ρ Oph A. The already published XMM–Newton data have been re-analysed showing that the X-ray spectra of ρ Oph A are compatible with the presence of a non-thermal X-ray component. We discuss a scenario where the emission phenomena occurring at the extremes of the electromagnetic spectrum, radio and X-ray, are directly induced by the same plasma process. We interpret the observed X-ray and radio features of ρ Oph A as having an auroral origin.
ABSTRACT
The non-thermal radio emission of main-sequence early-type stars is a signature of stellar magnetism. We present multiwavelength (1.6–16.7 GHz) ATCA measurements of the early-type magnetic ...star ρ OphC, which is a flat-spectrum non-thermal radio source. The ρ OphC radio emission is partially circularly polarized with a steep spectral dependence: the fraction of polarized emission is about $60{{\ \rm per\ cent}}$ at the lowest frequency sub-band (1.6 GHz) while is undetected at 16.7 GHz. This is clear evidence of coherent Auroral Radio Emission (ARE) from the ρ OphC magnetosphere. Interestingly, the detection of the ρ OphC’s ARE is not related to a peculiar rotational phase. This is a consequence of the stellar geometry, which makes the strongly anisotropic radiation beam of the amplified radiation always pointed towards Earth. The circular polarization sign evidences mainly amplification of the ordinary mode of the electromagnetic wave, consistent with a maser amplification occurring within dense regions. This is indirect evidence of the plasma evaporation from the polar caps, a phenomenon responsible for the thermal X-ray aurorae. ρ OphC is not the first early-type magnetic star showing the O-mode dominated ARE but is the first star with the ARE always on view.
ABSTRACT
Supernova remnants (SNRs) represent a powerful laboratory to study the cosmic ray acceleration processes at shocks, and their relation to the properties of the circumstellar medium. With the ...aim of studying the high-frequency radio emission and investigating the energy distribution of accelerated electrons and the magnetic field conditions, we performed single-dish observations of the large and complex Cygnus Loop SNR from 7.0–24.8 GHz with the Medicina and Sardinia Radio Telescopes, focusing on the northern filament (NGC 6992) and the southern shell. Both regions show a spectrum well fitted by a power-law function (S ∝ ν−α), with spectral index α = 0.45 ± 0.05 for NGC 6992 and α = 0.49 ± 0.01 for the southern shell and without any indication of a spectral break. The spectra are significantly flatter than the whole Cygnus Loop spectrum (α = 0.54 ± 0.01), suggesting a departure from the plain shock acceleration mechanisms, which for NGC 6992 could be related to the ongoing transition towards a radiative shock. We model the integrated spectrum of the whole SNR considering the evolution of the maximum energy and magnetic field amplification. Through the radio spectral parameters, we infer a magnetic field at the shock of 10 μG. This value is compatible with purely adiabatic compression of the interstellar magnetic field, suggesting that the amplification process is currently inefficient.
We present a comprehensive statistical analysis of star-forming objects located in the vicinities of 1360 bubble structures throughout the Galactic plane and their local environments. The compilation ...of ~70 000 star-forming sources, found in the proximity of the ionized (Hii) regions and detected in both Hi-GAL and GLIMPSE surveys, provided a broad overview of the different evolutionary stages of star-formation in bubbles, from prestellar objects to more evolved young stellar objects (YSOs). Surface density maps of star-forming objects clearly reveal an evolutionary trend where more evolved star-forming objects (Class II YSO candidates) are found spatially located near the center, while younger star-forming objects are found at the edge of the bubbles. We derived dynamic ages for a subsample of 182 H ii regions for which kinematic distances and radio continuum flux measurements were available. We detect approximately 80% more star-forming sources per unit area in the direction of bubbles than in the surrounding fields. We estimate the clump formation efficiency (CFE) of Hi-GAL clumps in the direction of the shell of the bubbles to be ~15%, around twice the value of the CFE in fields that are not affected by feedback effects. We find that the higher values of CFE are mostly due to the higher CFE of protostellar clumps, in particular in younger bubbles, whose density of the bubble shells is higher. We argue that the formation rate from prestellar to protostellar phase is probably higher during the early stages of the (H ii ) bubble expansion. Furthermore, we also find a higher fraction of massive YSOs (MYSOs) in bubbles at the early stages of expansion (<2 Myr) than older bubbles. Evaluation of the fragmentation time inside the shell of bubbles advocates the preexistence of clumps in the medium before the bubble expansion in order to explain the formation of MYSOs in the youngest H ii regions (<1 Myr), as supported by numerical simulations. Approximately 23% of the Hi-GAL clumps are found located in the direction of a bubble, with 15% for prestellar clumps and 41% for protostellar clumps. We argue that the high fraction of protostellar clumps may be due to the acceleration of the star-formation process cause by the feedback of the (Hii) bubbles.
ABSTRACT
We present observations of a region of the Galactic plane taken during the Early Science Program of the Australian Square Kilometre Array Pathfinder (ASKAP). In this context, we observed the ...scorpio field at 912 MHz with an uncompleted array consisting of 15 commissioned antennas. The resulting map covers a square region of ∼40 deg2, centred on (l, b) = (343.5°, 0.75°), with a synthesized beam of 24 × 21 arcsec2 and a background rms noise of 150–200 μJy beam−1, increasing to 500–600 μJy beam−1 close to the Galactic plane. A total of 3963 radio sources were detected and characterized in the field using the caesar source finder. We obtained differential source counts in agreement with previously published data after correction for source extraction and characterization uncertainties, estimated from simulated data. The ASKAP positional and flux density scale accuracy were also investigated through comparison with previous surveys (MGPS, NVSS) and additional observations of the scorpio field, carried out with ATCA at 2.1 GHz and 10 arcsec spatial resolution. These allowed us to obtain a measurement of the spectral index for a subset of the catalogued sources and an estimated fraction of (at least) 8 per cent of resolved sources in the reported catalogue. We cross-matched our catalogued sources with different astronomical data bases to search for possible counterparts, finding ∼150 associations to known Galactic objects. Finally, we explored a multiparametric approach for classifying previously unreported Galactic sources based on their radio-infrared colours.
Abstract
We present ALMA band 6 observations of the luminous blue variable
η
Car obtained within the ALMAGAL program. We report SiO
J
= 5 → 4, SiS
J
= 12 → 11, and SiN
N
= 5 → 4 emission in the ...equatorial region of the Homunculus nebula, constituting the first detection of silicon- and sulfur-bearing molecules in the outskirts of a highly evolved, early-type massive star. The SiO, SiS, and SiN trace a clumpy equatorial ring that surrounds the central binary at a projected distance of ∼2″, delineating the inner rims of the butterfly-shaped dusty region. The formation of silicon-bearing compounds is presumably related to the continuous recycling of dust due to the variable wind regime of
η
Car, which destroys grains and releases silicon back to the gas phase. We discuss possible formation routes for the observed species, contextualizing them within the current molecular inventory of
η
Car. We find that the SiO and SiS fractional abundances in localized clumps of the ring, 6.7 × 10
−9
and 1.2 × 10
−8
, respectively, are exceptionally lower than those measured in C- and O-rich AGB stars and cool supergiants, while the higher SiN abundance, 3.6 × 10
−8
, evidences the nitrogen-rich chemistry of the ejecta. These abundances must be regarded as strict upper limits, since the distribution of H
2
in the Homunculus is unknown. In any case, these findings shed new light on the peculiar molecular ecosystem of
η
Car and establish its surroundings as a new laboratory to investigate the life cycle of silicate dust in extreme astrophysical conditions.
ABSTRACT
Low- and intermediate-mass stars end their life dispersing their outer layers into the circumstellar medium, during the asymptotic and post-asymptotic giant branch phases. OH masers at 18 cm ...offer an effective way to probe their circumstellar environment. In this work, we present the discovery of seven OH maser sources likely associated with such evolved stars from the visual inspection of Australian SKA Pathfinder (ASKAP) continuum images. These seven sources do not emit real continuum emission, but the high sensitivity of our images allows us to detect their maser emission, resembling continuum sources. To confirm their nature, we carried out spectral-line observations with ATCA. All the sources showed the double-peaked spectra at 1612 MHz, typical of evolved stars. The detection of maser emission in continuum images can be a complementary and easy-to-use method to discover new maser sources with the large-area deep surveys conducted with the Square Kilometre Array (SKA) precursors. The implication for radio stars studies are remarkable since pure OH maser sources (i.e. with no continuum associated) represent, at a sensitivity of $100\,\mu \mathrm{Jy\,beam}^{-1}$, about 4 per cent of all Galactic sources and by far the most numerous stellar population.
ABSTRACT
In the broad context of the Australian Square Kilometer Array Pathfinder (ASKAP) early-science phase and preparation for the related surveys, we report the first radio observations towards ...the Galactic plane. The targeted field was chosen to encompass the entire SCORPIO survey, one of the several pathfinder projects for the Evolutionary Map of the Universe survey planned with the ASKAP. The observations were carried out in 2018 January at a central frequency of 912 MHz, with 15 operational antennas, and covered a total area of about 40 square degrees in three different pointings. The final image has a resolution of 24.1 × 21.1 arcsec2 and a median rms of $541\ \mu \mathrm{Jy\ beam}^{-1}$. We were able to extract 3545 candidate sources, 75 per cent of them point sources. For a preliminary validation, a comparison with the 843 MHz Molonglo Galactic Plane Survey is presented. Although the present observations were obtained with the ASKAP only partially deployed, its unique capability to map complex sources, such as those inhabiting the Galactic plane, at different angular scales, is highlighted. Within the SCORPIO field all the previously classified H ii regions, Planetary Nebulae (PNe), and supernovae remnants (SNRs), previously known to be radio sources, were detected. We also report new radio detections from several H ii regions previously classified as ‘candidates’ or ‘radio quiet’ and from half of all the PNe in the SCORPIO field with robust classification. Most notably, we find numerous unclassified, extended sources which constitute a promising sample of candidates H ii regions and SNRs.
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