arXiv subjects

Fabrice Herpin

Publications and source records attributed to Fabrice Herpin.

16 recordsLinked to original sources

arXiv2025

Confirming the Magnetic Field Detection at the Surface of $χ$ Cyg

We present spectropolarimetric observations of $χ$ Cygni obtained with Neo-Narval at Télescope Bernard Lyot in 2025. We obtained observations across three epochs (2025 Jul, Aug, and Oct) near maximum light to search for magnetic field signatures at the stellar photosphere. We detected a clear circular polarization signal in the 2025 Aug observations (pulsation phases $0.99$ to $0.01$). We measure a mean longitudinal magnetic field of $B_l = 3.4 \pm 0.6$ G. No detections were obtained for the 2025 Jul and Oct epochs. The pulsation-phase dependence suggests that field detection is tied to specific shock conditions near maximum light.

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arXiv2025

Evidence for the Keplerian orbit of a close companion around a giant star

Close companions influence stellar evolution through tidal interactions, mass transfer, and mass loss effects. While such companions are detected around young stellar objects, main-sequence stars, red giants, and compact objects, direct observational evidence of close-in companions around asymptotic giant branch (AGB) stars has remained elusive. Here, we present (sub)millimeter time-domain imaging spectroscopy revealing the Keplerian motion of a close-in companion around the AGB star pi1 Gruis. The companion, slightly more massive than the AGB star, is likely a main-sequence star. Unlike more evolved stars with companions at comparable distances, pi1 Gru's companion follows a circular orbit, suggesting an eccentricity-generating mechanism late- or post-AGB. Our analysis suggests that model-predicted circularization rates may be underestimated. Our results highlight the potential of multi-epoch (sub)millimeter interferometry in detecting the Keplerian motion of close companions to giant stars and open avenues for our understanding of tidal interaction physics and binary evolution.

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arXiv2021

Atomium: The astounding complexity of the near circumstellar environment of the M-type AGB star R Hydrae. I. Morpho-kinematical interpretation of CO and SiO emission

Evolved low- to intermediate-mass stars are known to shed their gaseous envelope into a large, dusty, molecule-rich circumstellar nebula which typically develops a high degree of structural complexity. Most of the large-scale, spatially correlated structures in the nebula are thought to originate from the interaction of the stellar wind with a companion. As part of the Atomium large programme, we observed the M-type asymptotic giant branch (AGB) star R Hydrae with ALMA. The morphology of the inner wind of R Hya, which has a known companion at ~3500 au, was determined from maps of CO and SiO obtained at high angular resolution. A map of the CO emission reveals a multi-layered structure consisting of a large elliptical feature at an angular scale of ~10'' that is oriented along the north-south axis. The wind morphology within the elliptical feature is dominated by two hollow bubbles. The bubbles are on opposite sides of the AGB star and lie along an axis with a position angle of ~115 deg. Both bubbles are offset from the central star, and their appearance in the SiO channel maps indicates that they might be shock waves travelling through the AGB wind. An estimate of the dynamical age of the bubbles yields an age of the order of 100 yr, which is in agreement with the previously proposed elapsed time since the star last underwent a thermal pulse. When the CO and SiO emission is examined on subarcsecond angular scales, there is evidence for an inclined, differentially rotating equatorial density enhancement, strongly suggesting the presence of a second nearby companion. The position angle of the major axis of this disc is ~70 deg in the plane of the sky. We tentatively estimate that a lower limit on the mass of the nearby companion is ~0.65 Msol on the basis of the highest measured speeds in the disc and the location of its inner rim at ~6 au from the AGB star.

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arXiv2020

Atomium: A high-resolution view on the highly asymmetric wind of the AGB star Pi1 Gruis. I. First detection of a new companion and its effect on the inner wind

The nebular circumstellar environments of cool evolved stars are known to harbour a rich morphological complexity of gaseous structures on different length scales. A large part of these density structures are thought to be brought about by the interaction of the stellar wind with a close companion. The S-type asymptotic giant branch star Pi1 Gruis, which has a known companion at ~440 au and is thought to harbour a second, closer-by (<10 au) companion, was observed with the Atacama Large Millimeter/submillimeter Array as part of the ATOMIUM Large programme. In this work, the brightest CO, SiO, and HCN molecular line transitions are analysed. The continuum map shows two maxima, separated by 0.04'' (6 au). The CO data unambiguously reveal that Pi1 Gru's circumstellar environment harbours an inclined, radially outflowing, equatorial density enhancement. It contains a spiral structure at an angle of 38+/-3 deg with the line-of-sight. The HCN emission in the inner wind reveals a clockwise spiral, with a dynamical crossing time of the spiral arms consistent with a companion at a distance of 0.04'' from the asymptotic giant branch star, which is in agreement with the position of the secondary continuum peak. The inner wind dynamics imply a large acceleration region, consistent with a beta-law power of ~6. The CO emission suggests that the spiral is approximately Archimedean within 5'', beyond which this trend breaks down as the succession of the spiral arms becomes less periodic. The SiO emission at scales smaller than 0.5'' exhibits signatures of gas in rotation, which is found to fit the expected behaviour of gas in the wind-companion interaction zone. An investigation of SiO maser emission reveals what could be a stream of gas accelerating from the surface of the AGB star to the companion. Using these dynamics, we have tentatively derived an upper limit on the companion mass to be ~1.1 Msol.

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arXiv2019

Multi-line Herschel/HIFI observations of water reveal infall motions and chemical segregation around high-mass protostars

(Abridged) We use HIFI maps of the 987 GHz H2O 2(02)-1(11) emission to measure the sizes and shapes of 19 high-mass protostellar envelopes. To identify infall, we use HIFI spectra of the optically thin C18O 9-8 and H2O-18 1(11)-0(00) lines. The high-J C18O line traces the warm central material and redshifted H2O-18 1(11)-0(00) absorption indicates material falling onto the warm core. We probe small-scale chemical differentiation by comparing H2O 752 and 987 GHz spectra with those of H2O-18. Our measured radii of the central part of the H2O 2(02)-1(11) emission are 30-40% larger than the predictions from spherical envelope models, and axis ratios are <2, which we consider good agreement. For 11 of the 19 sources, we find a significant redshift of the H2O-18 1(11)-0(00) line relative to C18O 9-8. The inferred infall velocities are 0.6-3.2 km/s, and estimated mass inflow rates range from 7e-5 to 2e-2 M0/yr, with the highest mass inflow rates occurring toward the sources with the highest masses, and possibly the youngest ages. The other sources show either expanding motions or H2O-18 lines in emission. The H2O-18 1(11)-0(00) line profiles are remarkably similar to the "differences" between the H2O 2(02)-1(11) and 2(11)-2(02) profiles, suggesting that the H2O-18 line and the H2O 2(02)-1(11) absorption originate just inside the radius where water evaporates from grains, typically 1000-5000 au from the center. In some sources, the H2O-18 line is detectable in the outflow, where no C18O emission is seen. Together, the H2O-18 absorption and C18O emission profiles show that the water abundance around high-mass protostars has at least three levels: low in the cool outer envelope, high within the 100 K radius, and very high in the outflowing gas. Thus, despite the small regions, the combination of lines presented here reveals systematic inflows and chemical information about the outflows.

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arXiv2014

The physical conditions in IRDC clumps from Herschel HIFI observations of H2O

Context. The earliest phases of high-mass star formation are poorly understood. Aims. Our goal is to determine the physical conditions and kinematic structure of massive star-forming cloud clumps. Methods. We analyze H$_2$O 557 GHz line profiles observed with HIFI toward four positions in two infrared-dark cloud clumps . By comparison with ground-based C$^{17}$O, N$_2$H$^+$, CH$_3$OH and NH$_3$ line observations, we constrain the volume density and kinetic temperature of the gas and estimate the column density and abundance of H$_2$O and N$_2$H$^+$. Results. The observed water lines are complex with emission and absorption components. The absorption is red shifted and consistent with a cold envelope, while the emission is interpreted as resulting from protostellar outflows. The gas density in the clumps is $\sim$ 10$^7$ cm$^{-3}$. The o-H2O outflow column density is 0.3 to 3.0 10$^{14}$ cm$^{-2}$, the o-H2O absorption column density is between 1.5 10$^{14}$ and 2.6 10$^{15}$ cm$^-2$ with cold o-H2O abundances between 1.5 10$^{-9}$ and 3.1 10$^{-8}$. Conclusions. All clumps have high gas densities ($\sim$ 10$^7$ cm$^{-3}$) and display infalling gas. Three of the four clumps have outflows. The clumps form an evolutionary sequence as probed by H$_2$O N$_2$H$^+$, NH$_3$ and CH$_3$OH. We find that G28-MM is the most evolved, followed by G11-MM then G28-NH3. The least evolved clump is G11-NH3 which shows no sign-posts of star-formation. G11-NH3 is a high-mass pre-stellar core.

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arXiv2013

Probing the Turbulent Ambipolar Diffusion Scale in Molecular Clouds with Spectroscopy

We estimate the turbulent ambipolar diffusion length scale and magnetic field strength in the massive dense cores CygX-N03 and CygX-N53, located in the Cygnus-X star-forming region. The method we use requires comparing the velocity dispersions in the spectral line profiles of the coexistent ion and neutral pair H13CN and H13CO+ (J=1-0) at different length scales. We fit Kolmogorov-type power laws to the lower envelopes of the velocity dispersion spectra of the two species. This allows to calculate the turbulent ambipolar diffusion scale, which in turn determines the plane-of-the-sky magnetic field strength. We find turbulent ambipolar diffusion length scales of 3.8+-0.1 mpc and 21.2+-0.4 mpc, and magnetic field strengths of 0.33 mG and 0.76 mG for CygX-N03 and CygX-N53, respectively. These magnetic field values have uncertainties of a factor of a few. Despite a lower signal-to-noise ratio of the data in CygX-N53 than in CygX-N03, and the caveat that its stronger field might stem in part from projection effects, the difference in field strengths suggests different fragmentation activities of the two cores. Even though the quality of our data, obtained with the IRAM Plateau de Bure Interferometer (PdBI), is somewhat inferior to previous single-dish data, we demonstrate that this method is suited also for observations at high spatial resolution.

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arXiv2013

Characterization of the MALT90 Survey and the Mopra Telescope at 90 GHz

We characterize the Millimeter Astronomy Legacy Team 90 GHz (MALT90) Survey and the Mopra telescope at 90 GHz. We combine repeated position-switched observations of the source G300.968+01.145 with a map of the same source in order to estimate the pointing reliability of the position-switched observations and, by extension, the MALT90 survey; we estimate our pointing uncertainty to be 8 arcseconds. We model the two strongest sources of systematic gain variability as functions of elevation and time-of-day and quantify the remaining absolute flux uncertainty. Corrections based on these two variables reduce the scatter in repeated observations from 12-25% down to 10-17%. We find no evidence for intrinsic source variability in G300.968+01.145. For certain applications, the corrections described herein will be integral for improving the absolute flux calibration of MALT90 maps and other observations using the Mopra telescope at 90 GHz.

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arXiv2013

Water in star-forming regions with Herschel (WISH). IV. A survey of low-J H2O line profiles toward high-mass protostars

To understand the origin of water line emission and absorption during high-mass star formation, we decompose high-resolution Herschel-HIFI line spectra toward 19 high-mass star-forming regions into three distinct physical components. Protostellar envelopes are usually seen as narrow absorptions or emissions in the H2O 1113 and 1669 GHz ground-state lines, the H2O 987 GHz excited-state line, and the H2O-18 1102 GHz ground-state line. Broader features due to outflows are usually seen in absorption in the H2O 1113 and 1669 GHz lines, in 987 GHz emission, and not seen in H2O-18, indicating a low column density and a high excitation temperature. The H2O 1113 and 1669 GHz spectra show narrow absorptions by foreground clouds along the line of sight, which have a low column density and a low excitation temperature, although their H2O ortho/para ratios are close to 3. The intensities of the H2O 1113 and 1669 GHz lines do not show significant trends with luminosity, mass, or age. In contrast, the 987 GHz line flux increases with luminosity and the H2O-18 line flux decreases with mass. Furthermore, appearance of the envelope in absorption in the 987 GHz and H2O-18 lines seems to be a sign of an early evolutionary stage. We conclude that the ground state transitions of H2O trace the outer parts of the envelopes, so that the effects of star formation are mostly noticeable in the outflow wings. These lines are heavily affected by absorption, so that line ratios of H2O involving the ground states must be treated with caution. The average H2O abundance in high-mass protostellar envelopes does not change much with time. The 987 GHz line appears to be a good tracer of the mean weighted dust temperature of the source, which may explain why it is readily seen in distant galaxies.

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arXiv2010

Herschel-HIFI observations of H2O in high-mass star-forming regions: first results

This paper reviews the first results of observations of H2O line emission with Herschel-HIFI towards high-mass star-forming regions, obtained within the WISH guaranteed time program. The data reveal three kinds of gas-phase H2O: `cloud water' in cold tenuous foreground clouds, `envelope water' in dense protostellar envelopes, and `outflow water' in protostellar outflows. The low H2O abundance (1e-10 -- 1e-9) in foreground clouds and protostellar envelopes is due to rapid photodissociation and freeze-out on dust grains, respectively. The outflows show higher H2O abundances (1e-7 -- 1e-6) due to grain mantle evaporation and (probably) neutral-neutral reactions.

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arXiv2009

S-bearing molecules in Massive Dense Cores

Chemical composition of the massive cores forming high-mass stars can put some constrains on the time scale of the massive star formation: sulphur chemistry is of specific interest due to its rapid evolution in warm gas and because the abundance of sulphur bearing species increases significantly with the temperature. Two mid-infrared quiet and two brighter massive cores are observed in various transitions (E_up up to 289K) of CS, OCS, H2S, SO, SO2 and of their isotopologues at mm wavelengths with the IRAM 30m and CSO telescopes. 1D modeling of the dust continuum is used to derive the density and temperature laws, which are then applied in the RATRAN code to model the observed line emission, and to derive the relative abundances of the molecules. All lines, except the highest energy SO2 transition, are detected. Infall (up to 2.9km/s) may be detected towards the core W43MM1. The inferred mass rate is 5.8-9.4 10^{-2} M_{\odot}/yr. We propose an evolutionary sequence of our sources (W43MM1-IRAS18264-1152-IRAS05358+3543-IRAS18162-2048), based on the SED analysis. The analysis of the variations in abundance ratios from source to source reveals that the SO and SO2 relative abundances increase with time, while CS and OCS decrease. Molecular ratios, such as [OCS/H2S], [CS/H2S], [SO/OCS], [SO2/OCS], [CS/SO] and [SO2/SO] may be good indicators of evolution depending on layers probed by the observed molecular transitions. Observations of molecular emission from warmer layers, hence involving higher upper energy levels are mandatory to include.

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arXiv2009

Precessing planetary magnetospheres in SiO stars ? First detection of quasi-periodic polarization fluctuations in R Leo and V Cam

The origin of magnetism around AGB stars is uncertain. If these stars drive a dynamo, it cannot be sufficient to generate a strong global field, otherwise the observed X-ray luminosities would be higher. Other explanations for the circular polarization of SiO masers in the atmospheres are needed. The interaction of the AGB wind with previously ejected matter and with planets bears complex magneto-hydrodynamic phenomena on a short time scale, such that strong magnetic fields can be maintained locally. Here we provide observational evidence for the corresponding magnetic fluctuations. We trace magnetic activity with the circular polarization of the v=1, J=2-1 SiO masers, using a correlation polarimeter. In V Cam and R Leo, we find evidence of pseudo-periodic fluctuations of the circular polarization on a timescale of a few hours, from which we infer magnetic fluctuations of ~1G. The phenomenon is rare and restricted to a narrow range of velocities. It seems to be associated with planetary wake flows suggested by VLBI maps. While scenarios involving magnetic activity in the extended stellar atmosphere have problems to explain all observed features, precessing Jovian magnetospheres predict all of them. For the case of R Leo, we constrain the orbit of the planet (estimated period 5.2 years) and estimate a stellar mass of 0.7 solar masses.

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arXiv2006

Full polarization study of SiO masers at 86 GHz

We study the polarization of the SiO maser emission in a representative sample of evolved stars in order to derive an estimate of the strength of the magnetic field, and thus determine the influence of this magnetic field on evolved stars. We made simultaneous spectroscopic measurements of the 4 Stokes parameters, from which we derived the circular and linear polarization levels. The observations were made with the IF polarimeter installed at the IRAM 30m telescope. A discussion of the existing SiO maser models is developed in the light of our observations. Under the Zeeman splitting hypothesis, we derive an estimate of the strength of the magnetic field. The averaged magnetic field varies between 0 and 20 Gauss, with a mean value of 3.5 Gauss, and follows a 1/r law throughout the circumstellar envelope. As a consequence, the magnetic field may play the role of a shaping, or perhaps collimating agent of the circumstellar envelopes in evolved objects.

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arXiv2005

Water in the envelopes and disks around young high-mass stars

Single-dish spectra and interferometric maps of (sub)mm lines of H2O-18 and HDO are used to study the chemistry of water in eight regions of high-mass star formation. The spectra indicate HDO excitation temperatures of ~110 K and column densities in an 11'' beam of ~2x10^14 cm^-2 for HDO and ~2x10^17 cm^-2 for H2O, with the N(HDO)/N(H2O) ratio increasing with decreasing temperature. Simultaneous observations of CH3OH and SO2 indicate that 20-50% of the single-dish line flux arises in the molecular outflows of these objects. The outflow contribution to the H2O-18 and HDO emission is estimated to be 10-20%. Radiative transfer models indicate that the water abundance is low (~10^-6) outside a critical radius corresponding to a temperature in the protostellar envelope of ~100 K, and `jumps' to H2O/H2 ~ 10^-4 inside this radius. This value corresponds to the observed abundance of solid water and together with the derived HDO/H2O abundance ratios of ~1/1000 suggests that the origin of the observed water is evaporation of grain mantles. This idea is confirmed in the case of AFGL 2591 by interferometer observations of HDO, H2O-18 and SO2 lines, which reveal compact (~800 AU) emission with a systematic velocity gradient. This size is similar to that of the 1.3 mm continuum towards AFGL 2591, from which we estimate a mass of ~0.8 M0, or ~5% of the mass of the central star. We speculate that we may be observing a circumstellar disk in an almost face-on orientation.

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arXiv2004

Molecular hydrogen beyond the optical edge of an isolated spiral galaxy

We know little about the outermost portions of galaxies because there is little light coming from them. We do know that in many cases atomic hydrogen (HI) extends well beyond the optical radius \cite{Casertano91}. In the centers of galaxies, however, molecular hydrogen (H2) usually dominates by a large factor, raising the question of whether H2 is abundant also in the outer regions but hitherto unseen.Here we report the detection of emission from carbon monoxide (CO), the most abundant tracer of H2, beyond the optical radius of the nearby galaxy NGC 4414. The molecular clouds probably formed in the regions of relatively high HI column density and in the absence of spiral density waves. The relative strength of the lines from the two lowest rotational levels indicates that both the temperature and density of the H2 are quite low compared to conditions closer to the center. The inferred surface density of the molecular material continues the monotonic decrease from the inner regions. We conclude that while molecular clouds can form in the outer region of this galaxy, there is little mass associated with them.

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arXiv2004

HCN and HCO+ emission in the disk of M31

We report observations made with the IRAM 30m radiotelescope in the HCN(1-0) and HCO+(1-0) lines towards a sample of molecular complexes (GMCs) in the disk of the Andromeda galaxy (M31). The targets were identified bright CO GMCs selected from the IRAM 30m CO survey with various morphologies and environments. The clouds vary in galactocentric distances from 2.4 to 15.5kpc. The HCN and HCO+ emission is easily detected in almost all observed positions, with line widths generally similar to the CO ones and there is a good correlation between the two dense gas tracers. The HCO+ emission is slightly stronger than the HCN, in particular towards GMCs with a strong star formation activity. However the HCO+ emission is weaker than the HCN towards a quiescent cloud in the inner part of M31, which could be due to a lower abundance of HCO+. We derive I_HCN/I_CO ratios between 0.008 and 0.03 and I_HCO+/I_CO ratios between less than 0.003 and 0.04. We study the radial distribution of the dense gas in the disk of M31. Unlike our Galaxy the HCO+/CO ratio is lower in the center of M31 than in the arms, which can be explained by both a lower abundance of HCO+ and different conditions of excitation. Furthermore the HCN/CO and HCO+/CO ratios appear to be higher in the inner spiral arm and weaker in the outer arm.

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