arXiv subjects

C. Agliozzo

Publications and source records attributed to C. Agliozzo.

At least 19 recordsLinked to original sources

arXiv2021

LBV phenomenon and binarity: The environment of HR Car

Luminous blue variable stars (LBVs) are of great interest in massive-star evolution as they experience very high mass-loss episodes within short periods of time. HR Car is a famous member of this class in the Galaxy. It has a large circumstellar nebula and has also been confirmed as being in a binary system. One means of gaining information about the evolutionary status and physical nature of LBVs is studying their environments. We investigated the stellar content within ~100 pc of HR Car and also its circumstellar nebula. Very Large Telescope (VLT) Multi Unit Spectroscopic Explorer (MUSE) observations of a 2'x2' region around the star highlight the incompleteness of stellar classification for stars with magnitudes of V > 13 mag. Eight B0 to B9 stars have been identified which may lie in close spatial vicinity to HR Car. For a region with a radius of r =1.2 degree (~100 pc at a distance of 4.8 kpc) around HR Car, existing catalogs list several late O-type and early B-type stars, but only one early O-type star. Given the relatively low stellar and nebular masses in the HR Car system, no early O-type stars and only a few late O-type stars would be expected in association with HR Car. Instead, HR Car's location in a point vector diagram suggests that HR Car is not isolated, but is part of a moving group with a population of B-type stars in a spiral arm, and it has not received a strong kick from a supernova explosion of a companion star or a merger event. Potential binary evolution pathways for the HR Car system cannot be fully explored because of the unknown nature of the companion star. Furthermore, the MUSE observations reveal the presence of a fast outflow and "bullets" that have been ejected at intervals of about 400 years. These features may have been caused by recurrent mass transfer in the system.

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arXiv2021

The contribution by luminous blue variable stars to the dust content of the Magellanic Clouds

(Shortened) Luminous blue variable stars (LBVs) form dust as a result of episodic, violent mass loss. To investigate their contribution as dust producers in the Magellanic Clouds, we analyse 31 LBVs from a recent census. We built a maximally complete multi-wavelength dataset of these sources from archival data from near-IR to millimetre wavelengths. We review the LBV classification on the basis of the IR SED. To derive characteristic dust parameters, we fitted the photometry resulting from a stacking analysis. For comparison we also stacked the images of low- and intermediate-mass evolved stars in the LMC. We find four classes of sources: 1) LBVs showing mid-IR dust emission plus near-IR free-free emission from an ionised stellar wind (Class 1a) or only mid-IR dust emission (Class 1b); 2) LBVs with a near-IR excess due to free-free emission only (Class 2); 3) objects with an sgB[e] classification; and 4) objects with no detected stellar winds and no circumstellar matter in their SEDs. From the stacking analysis of the 18 Class 1 and 2 objects in the LMC, we derived an integrated dust mass of $0.11^{+0.06}_{-0.03} M_\odot$. This is two orders of magnitude larger than the value inferred from stacking 1342 extreme-AGB stars. The dust mass of individual LBVs does not correlate with the stellar parameters, possibly suggesting that the dust production mechanism is independent of the initial stellar mass or that the stars have different evolutionary histories. The total dust yield from LBVs over the age of the LMC is $\sim 10^4-10^5 M_\odot$. LBVs are potentially the second most important source of dust in normal galaxies. The role of dust destruction in LBV nebulae by a possible subsequent SN blast wave has yet to be determined. Recent theoretical developments in the field of dust processing by SN shocks highlight the potential survival of dust from the pre-existing circumstellar nebula.

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arXiv2021

A scaling relationship for non-thermal radio emission from ordered magnetospheres: from the top of the Main Sequence to planets

In this paper, we present the analysis of incoherent non-thermal radio emission from a sample of hot magnetic stars, ranging from early-B to early-A spectral type. Spanning a wide range of stellar parameters and wind properties, these stars display a commonality in their radio emission which presents new challenges to the wind scenario as originally conceived. It was thought that relativistic electrons, responsible for the radio emission, originate in current sheets formed where the wind opens the magnetic field lines. However, the true mass-loss rates from the cooler stars are too small to explain the observed non-thermal broadband radio spectra. Instead, we suggest the existence of a radiation belt located inside the inner-magnetosphere, similar to that of Jupiter. Such a structure explains the overall indifference of the broadband radio emissions on wind mass-loss rates. Further, correlating the radio luminosities from a larger sample of magnetic stars with their stellar parameters, the combined roles of rotation and magnetic properties have been empirically determined. Finally, our sample of early-type magnetic stars suggests a scaling relationship between the non-thermal radio luminosity and the electric voltage induced by the magnetosphere's co-rotation, which appears to hold for a broader range of stellar types with dipole-dominated magnetospheres (like the cases of the planet Jupiter and the ultra-cool dwarf stars and brown dwarfs). We conclude that well-ordered and stable rotating magnetospheres share a common physical mechanism for supporting the generation of non-thermal electrons.

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arXiv2020

The auroral radio emission of the magnetic B-type star rho OphC

The non-thermal radio emission of main-sequence early-type stars is a signature of stellar magnetism. We present multi-wavelength (1.6-16.7 GHz) ATCA measurements of the early-type magnetic star rho OphC, which is a flat-spectrum non-thermal radio source. The rho OphC radio emission is partially circularly polarized with a steep spectral dependence: the fraction of polarized emission is about 60% 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 rho OphC magnetosphere. Interestingly, the detection of the rho 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. rho 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.

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arXiv2020

Evidence for radio and X-ray auroral emissions from the magnetic B-type star rho Oph A

We present new ATCA multi-wavelength radio measurements (range 2.1-21.2 GHz) of the early-type magnetic star rho Oph A, performed in March 2019 during 3 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 rho Oph A at 2.1 GHz. We collected high-resolution optical spectra of rho Oph A acquired by several instruments over a time span of about ten years. We also report new magnetic field measurements of rho Oph A that, together with the radio light curves and the temporal variation of the equivalent width of the HeI line (lambda=5015 Angstrom), 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 rho Oph A's magnetosphere. Past XMM measurements showed periodic X-ray pulses from rho Oph A. We correlate the X-ray light curve with the magnetic field geometry of rho Oph A. The already published XMM data have been re-analyzed showing that the X-ray spectra of rho 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 rho Oph A as having an auroral origin.

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arXiv2019

Study of the Galactic radio sources in the SCORPIO survey resolved by ATCA at 2.1 GHz

We present a catalogue of a large sample of extended radio sources in the SCORPIO field, observed and resolved by the Australia Telescope Compact Array. SCORPIO, a pathfinder project for addressing the early operations of the Australia SKA Pathfinder, is a survey of ~5 square degrees between 1.4 and 3.1 GHz, centered at l=343.5°, b=0.75° and with an angular resolution of about 10 arcsec. It is aimed at understanding the scientific and technical challenges to be faced by future Galactic surveys. With a mean sensitivity around 100 $μ$Jy/beam and the possibility to recover angular scales at least up to 4 arcmin, we extracted 99 extended sources, 35 of them detected for the first time. Among the 64 known sources 55 had at least a tentative classification in literature. Studying the radio morphology and comparing the radio emission with infrared we propose as candidates 6 new H II regions, 2 new planetary nebulae, 2 new luminous blue variable or Wolf--Rayet stars and 3 new supernova remnants. This study provides an overview of the potentiality of future radio surveys in terms of Galactic source extraction and characterization and a discussion on the difficulty to reduce and analyze interferometric data on the Galactic plane.

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arXiv2019

Mid-infrared evolution of eta Car from 1968 to 2018

Eta Car is one of the most luminous and massive stars in our Galaxy and is the brightest mid-infrared (mid-IR) source in the sky, outside our solar system. Since the late 1990s the central source has dramatically brightened at ultraviolet and optical wavelengths. This might be explained by a decrease in circumstellar dust extinction. We aim to establish the mid-IR flux evolution and further our understanding of the star's ultraviolet and optical brightening. Mid-IR images from $8-20~μ$m were obtained in 2018 with VISIR at the Very Large Telescope. Archival data from 2003 and 2005 are retrieved from the ESO Science Archive Facility and historical records are collected from publications. We present the highest angular resolution mid-IR images of $η$ Car to date at the corresponding wavelengths ($\geq 0.22''$). We reconstruct the mid-IR evolution of the spectral energy distribution of the spatially integrated Homunculus nebula from 1968 to 2018 and find no long-term changes. Eta Car's bolometric luminosity has been stable over the past five decades. We do not observe a long-term decrease in the mid-IR flux densities that could be associated with the brightening at ultraviolet and optical wavelengths, but circumstellar dust must be declining in our line-of-sight only. Short-term flux variations within about 25% of the mean levels could be present.

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arXiv2018

The polarization mode of the auroral radio emission from the early-type star HD142301

We report the detection of the auroral radio emission from the early-type magnetic star HD142301. New VLA observations of HD142301 detected highly polarized amplified emission occurring at fixed stellar orientations. The coherent emission mechanism responsible for the stellar auroral radio emission amplifies the radiation within a narrow beam, making the star where this phenomenon occurs similar to a radio lighthouse. The elementary emission process responsible for the auroral radiation mainly amplifies one of the two magneto-ionic modes of the electromagnetic wave. This explains why the auroral pulses are highly circularly polarized. The auroral radio emission of HD142301 is characterized by a reversal of the sense of polarization as the star rotates. The effective magnetic field curve of HD142301 is also available making it possible to correlate the transition from the left to the right-hand circular polarization sense (and vice-versa) of the auroral pulses with the known orientation of the stellar magnetic field. The results presented in this letter have implications for the estimation of the dominant magneto-ionic mode amplified within the HD142301 magnetosphere.

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arXiv2018

Detection of Alpha Centauri at radio wavelengths: chromospheric emission and search for star-planet interaction

At radio wavelengths, solar-type stars emit thermal free-free and gyroresonance, gyrosynchrotron, and impulsive coherent emission. Thermal free-free emission originates at layers where the optical depth is close to unit, while high brightness temperature, variable emission, can be due to flares via gyrosynchrotron emission. We observed the alpha Cen system with the Australian Telescope Compact Array at 2 GHz for three days and 17 GHz for one day. Both stars have been detected at 17 GHz, while only an upper limit has been obtained at low frequency despite the longer integration time. The brightness temperatures are consistent with the temperature of the upper chromosphere of the Sun. Inverting the formulae of the free-free emission, the average electron density of the plasma has been inferred. The same procedure was applied to the data in the millimetre recently acquired with ALMA. A comparison with the atmospheric solar models reveals a higher level of activity in alpha Cen B rather than in alpha Cen A, even if still at quiescent level. The non detection at 2 GHz allows us to put a lower limit in the filling factor of active regions. The claimed detection of an Earth size planet in close orbit to alpha Cen B, although doubtful, opens the opportunity to check the existence of Star-Planet Magnetic Interaction (SPMI). We constructed dynamic spectra in the 1.3 - 2.9 GHz of the 2 stars to search for time-variable coherent emission but obtained a null result.

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arXiv2018

The lowest metallicity type II supernova from the highest mass red-supergiant progenitor

Red supergiants have been confirmed as the progenitor stars of the majority of hydrogen-rich type II supernovae. However, while such stars are observed with masses >25M$_\odot$, detections of >18M$_\odot$ progenitors remain elusive. Red supergiants are also expected to form at all metallicities, but discoveries of explosions from low-metallicity progenitors are scarce. Here, we report observations of the type II supernova, SN 2015bs, for which we infer a progenitor metallicity of $\leq$0.1Z$_\odot$ from comparison to photospheric-phase spectral models, and a Zero Age Main-Sequence mass of 17-25M$_\odot$ through comparison to nebular-phase spectral models. SN 2015bs displays a normal 'plateau' light-curve morphology, and typical spectral properties, implying a red supergiant progenitor. This is the first example of such a high mass progenitor for a 'normal' type II supernova, suggesting a link between high mass red supergiant explosions and low-metallicity progenitors.

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arXiv2018

A combined multiwavelength VLA/ALMA/Chandra study unveils the complex magnetosphere of the B-type star HR5907

We present new radio/millimeter measurements of the hot magnetic star HR5907 obtained with the VLA and ALMA interferometers. We find that HR5907 is the most radio luminous early type star in the cm-mm band among those presently known. Its multi-wavelength radio light curves are strongly variable with an amplitude that increases with radio frequency. The radio emission can be explained by the populations of the non-thermal electrons accelerated in the current sheets on the outer border of the magnetosphere of this fast rotating magnetic star. We classify HR5907 as another member of the growing class of strongly magnetic fast rotating hot stars where the gyro-synchrotron emission mechanism efficiently operates in their magnetospheres. The new radio observations of HR5907 are combined with archival X-ray data to study the physical condition of its magnetosphere. The X-ray spectra of HR5907 show tentative evidence for the presence of non-thermal spectral component. We suggest that non-thermal X-rays originate a stellar X-ray aurora due to streams of non-thermal electrons impacting on the stellar surface. Taking advantage of the relation between the spectral indices of the X-ray power-law spectrum and the non-thermal electron energy distributions, we perform 3-D modeling of the radio emission for HR5907. The wavelength-dependent radio light-curves probe magnetospheric layers at different heights above the stellar surface. A detailed comparison between simulated and observed radio light-curves leads us to conclude that the stellar magnetic field of HR5907 is likely non-dipolar, providing further indirect evidence of the complex magnetic field topology of HR5907.

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arXiv2017

A kilonova as the electromagnetic counterpart to a gravitational-wave source

Gravitational waves were discovered with the detection of binary black hole mergers and they should also be detectable from lower mass neutron star mergers. These are predicted to eject material rich in heavy radioactive isotopes that can power an electromagnetic signal called a kilonova. The gravitational wave source GW170817 arose from a binary neutron star merger in the nearby Universe with a relatively well confined sky position and distance estimate. Here we report observations and physical modelling of a rapidly fading electromagnetic transient in the galaxy NGC4993, which is spatially coincident with GW170817 and a weak short gamma-ray burst. The transient has physical parameters broadly matching the theoretical predictions of blue kilonovae from neutron star mergers. The emitted electromagnetic radiation can be explained with an ejected mass of 0.04 +/- 0.01 Msol, with an opacity of kappa <= 0.5 cm2/gm at a velocity of 0.2 +/- 0.1c. The power source is constrained to have a power law slope of beta = -1.2 +/- 0.3, consistent with radioactive powering from r-process nuclides. We identify line features in the spectra that are consistent with light r-process elements (90 < A < 140). As it fades, the transient rapidly becomes red, and emission may have contribution by a higher opacity, lanthanide-rich ejecta component. This indicates that neutron star mergers produce gravitational waves, radioactively powered kilonovae, and are a nucleosynthetic source of the r-process elements.

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arXiv2017

SCORPIO-II: Spectral indices of weak Galactic radio sources

In the next few years the classification of radio sources observed by the large surveys will be a challenging problem, and spectral index is a powerful tool for addressing it. Here we present an algorithm to estimate the spectral index of sources from multiwavelength radio images. We have applied our algorithm to SCORPIO (Umana et al. 2015), a Galactic Plane survey centred around 2.1 GHz carried out with ATCA, and found we can measure reliable spectral indices only for sources stronger than 40 times the rms noise. Above a threshold of 1 mJy, the source density in SCORPIO is 20 percent greater than in a typical extra-galactic field, like ATLAS (Norris et al. 2006), because of the presence of Galactic sources. Among this excess population, 16 sources per square degree have a spectral index of about zero, suggesting optically thin thermal emission such as Hii regions and planetary nebulae, while 12 per square degree present a rising spectrum, suggesting optically thick thermal emission such as stars and UCHii regions.

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arXiv2017

Spectroscopic and photometric oscillatory envelope variability during the S Doradus outburst of the Luminous Blue Variable R71

To better understand the LBV phenomenon, we analyze multi-epoch and multi-wavelength spectra and photometry of R71. Pre-outburst spectra are analyzed with the radiative transfer code CMFGEN to determine the star's fundamental stellar parameters. During quiescence, R71 has an effective temperature of $T_\mathrm{eff} = 15\,500~K$ and a luminosity of log$(L_*/L_{\odot})$ = 5.78 and is thus a classical LBV, but at the lower luminosity end of this group. We determine its mass-loss rate to $4.0 \times 10^{-6}~M_{\odot}~$yr$^{-1}$. We present R71's spectral energy distribution from the near-ultraviolet to the mid-infrared during its present outburst. Mid-infrared observations suggest that we are witnessing dust formation and grain evolution. Semi-regular oscillatory variability in the star's light curve is observed during the current outburst. Absorption lines develop a second blue component on a timescale twice that length. The variability may consist of one (quasi-)periodic component with P ~ 425/850 d with additional variations superimposed. During its current S Doradus outburst, R71 occupies a region in the HR diagram at the high-luminosity extension of the Cepheid instability strip and exhibits similar irregular variations as RV Tau variables. LBVs do not pass the Cepheid instability strip because of core evolution, but they develop comparable cool, low-mass, extended atmospheres in which convective instabilities may occur. As in the case of RV Tau variables, the occurrence of double absorption lines with an apparent regular cycle may be due to shocks within the atmosphere and period doubling may explain the factor of two in the lengths of the photometric and spectroscopic cycles.

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arXiv2017

The Luminous Blue Variable RMC127 as seen with ALMA and ATCA

We present ALMA and ATCA observations of the luminous blue variable \rmc. The radio maps show for the first time the core of the nebula and evidence that the nebula is strongly asymmetric with a Z-pattern shape. Hints of this morphology are also visible in the archival \emph{HST} $\rm Hα$ image, which overall resembles the radio emission. The emission mechanism in the outer nebula is optically thin free-free in the radio. At high frequencies, a component of point-source emission appears at the position of the star, up to the ALMA frequencies. The rising flux density distribution ($S_ν\sim ν^{0.78\pm0.05}$) of this object suggests thermal emission from the ionized stellar wind and indicates a departure from spherical symmetry with $n_{e}(r)\propto r^{-2}$. We examine different scenarios to explain this excess of thermal emission from the wind and show that this can arise from a bipolar outflow, supporting the suggestion by other authors that the stellar wind of \rmc is aspherical. We fit the data with two collimated ionized wind models and we find that the mass-loss rate can be a factor of two or more smaller than in the spherical case. We also fit the photometry obtained by IR space telescopes and deduce that the mid- to far-IR emission must arise from extended, cool ($\sim80\,\rm K$) dust within the outer ionized nebula. Finally we discuss two possible scenarios for the nebular morphology: the canonical single star expanding shell geometry, and a precessing jet model assuming presence of a companion star.

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arXiv2016

New ATCA, ALMA and VISIR observations of the candidate LBV SK-67266 (S61): the nebular mass from modelling 3D density distributions

We present new observations of the nebula around the Magellanic candidate Luminous Blue Variable S61. These comprise high-resolution data acquired with the Australia Telescope Compact Array (ATCA), the Atacama Large Millimetre/Submillimetre Array (ALMA), and VISIR at the Very Large Telescope (VLT). The nebula was detected only in the radio, up to 17 GHz. The 17 GHz ATCA map, with 0.8 arcsec resolution, allowed a morphological comparison with the H$α$ Hubble Space Telescope image. The radio nebula resembles a spherical shell, as in the optical. The spectral index map indicates that the radio emission is due to free-free transitions in the ionised, optically thin gas, but there are hints of inhomogeneities. We present our new public code RHOCUBE to model 3D density distributions, and determine via Bayesian inference the nebula's geometric parameters. We applied the code to model the electron density distribution in the S61 nebula. We found that different distributions fit the data, but all of them converge to the same ionised mass, ~0.1 $\rm M\odot$, which is an order of magnitude smaller than previous estimates. We show how the nebula models can be used to derive the mass-loss history with high-temporal resolution. The nebula was probably formed through stellar winds, rather than eruptions. From the ALMA and VISIR non-detections, plus the derived extinction map, we deduce that the infrared emission observed by space telescopes must arise from extended, diffuse dust within the ionised region.

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arXiv2016

Exploring the multifaceted circumstellar environment of the Luminous Blue Variable HR Carinae

We present a multi-wavelength study of the Galactic Luminous Blue Variable HR Carinae, based on new high resolution mid-infrared (IR) and radio images obtained with the Very Large Telescope (VLT) and the Australia Telescope Compact Array (ATCA), which have been complemented by far-infrared Herschel-PACS observations and ATCA archive data. The Herschel images reveal the large-scale distribution of the dusty emitting nebula, which extends mainly to the North-East direction, up to 70 arcsec from the central star, and is oriented along the direction of the space motion of the star. In the mid-infrared images, the brightness distribution is characterized by two arc-shaped structures, tracing an inner envelope surrounding the central star more closely. At radio wavelengths, the ionized gas emission lies on the opposite side of the cold dust with respect to the position of the star, as if the ionized front was confined by the surrounding medium in the North-South direction. The comparison with previous data indicates significant changes in the radio nebula morphology and in the mass-loss rate from the central star, which has increased from 6.1$\times10^{-6}$ M$_{\odot}$yr$^{-1}$ in 1994-1995 to $1.17\times10^{-5}$ M$_{\odot}$yr$^{-1}$ in 2014. We investigate possible scenarios that could have generated the complex circumstellar environment revealed by our multi-wavelength data.

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arXiv2015

Searching for OH maser emission towards the MIPSGAL compact Galactic bubbles

We conducted radio observations searching for OH 18-cm maser emission from a sample of 169 unclassified MIPSGAL compact Galactic bubbles. These sources are thought to be the circumstellar envelopes of different kinds of evolved stars. Our observations were aimed at shedding light on the nature of MIPSGAL bubbles, since their characterisation is a fundamental aid for the development of accurate physical models of stellar and Galaxy evolution. The maser emission is observatively linked to the last stages of the life of low- and intermediate-mass stars, which may constitute a significant fraction of the MIPSGAL bubbles. In particular OH masers are usually observed towards post-AGB stars. Our observations were performed with the Green Bank Telescope and, for each source, produced spectra around the four OH 18-cm transitions. The observations were compared with archive interferometer data in order to exclude possible contamination from nearby sources. The main result is that the OH maser emission is not a common feature among the MIPSGAL bubbles, with only one certain detection. We conclude that among the MIPSGAL bubbles the post-AGB stars could be very rare.

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