arXiv subjects

C. Goddi

Publications and source records attributed to C. Goddi.

At least 19 recordsLinked to original sources

arXiv2025

ALMA-IMF XX: Core fragmentation in the W51 high-mass star-forming region

We present a study of core fragmentation in the W51-E and W51-IRS2 protoclusters in the W51 high-mass star-forming region. The identification of core fragmentation is achieved by the spatial correspondence of cores and compact sources which are detected in the short (low resolution) and the long baseline (high resolution) continuum images with the Atacama Large Millimeter/submillimeter Array (ALMA) in Bands 3 (3 mm) and 6 (1.3 mm), respectively. We characterize the compact sources found in the long baseline image, and conclude that the compact sources are pre/protostellar objects (PPOs) that are either prestellar dust cores or dust disks or envelopes around protostars. The observed trend of core fragmentation in W51 is that (i) massive cores host more PPOs, (ii) bright PPOs are preferentially formed in massive cores, (iii) equipartition of flux between PPOs is uncommon. Thermal Jeans masses of parent cores are insufficient to explain the masses of their fragments, and this trend is more prominent at high-mass cores. We also find that unfragmented cores are large, less massive, and less dense than fragmented cores.

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arXiv2025

A compact Keplerian-like disc and H30$\alpha$ emission towards the radio jet in the massive protostar W75N(B)-VLA3

Studying young, high-mass stellar objects is challenging for testing models of massive star formation due to their great distances, often kiloparsecs away. This requires extremely high-angular resolution to resolve features like accretion discs around massive protostars. Consequently, while powerful, collimated outflows are evident in massive protostars, the compact accretion discs anticipated at their centres are still proving difficult to pinpoint. This study presents ALMA continuum and molecular line observations at 1.3 mm of the massive protostar W75N(B)-VLA3. The observations achieve an angular resolution of $\sim$0.12$^{\prime\prime}$ ($\sim$156 au). Dense gas tracers reveal a circumstellar disc of $\sim$450 au in radius surrounding VLA3, with an orientation perpendicular to its associated thermal radio jet. From the millimetre continuum, a total mass of $\approx$0.43-1.74 $M_\odot$ is estimated for the disc. The disc's velocity profile is consistent with Keplerian rotation around a protostar of $\approx$16 $M_\odot$. This adds VLA3 to the small number of massive disc-protostar-jet systems documented in the literature with a centrifugally supported disc with a radius less than 500 au. Additionally, we detected H30$\alpha$ recombination line emission towards the radio jet powered by VLA3. Despite limitations in the spatial and spectral resolution, our data reveal a very broad line, indicative of high-velocity motions, as well as a tentative velocity gradient in the jet's direction, thus favouring the H30$\alpha$ emission to originate from the radio jet. Should this interpretation be confirmed with new observations, W75N(B)-VLA3 could represent the first protostellar radio jet for which a thermal radio recombination line has been detected.

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arXiv2025

Survey of (sub)mm water masers in low-mass star-forming regions

Water masers are common in star-forming regions (SFRs), with the 22.235 GHz transition widely detected in both high- and low-mass protostars. In contrast, (sub)millimeter water maser transitions remain poorly studied, especially in low-mass SFRs. We search for millimeter water masers in a sample of low-mass SFRs previously known to exhibit 22 GHz emission. We target the transitions at 183.3, 321.2, and 325.2 GHz, respectively. We also examine their potential as probes of evolutionary stage by comparing them with previously reported Class I methanol masers (MM). We used the APEX 12m telescope. To assess the evolutionary stage of each source, we modeled their spectral energy distributions (SEDs) using archival data and used the derived dust temperatures as proxies of ages. We then compared the occurrence of water and methanol masers across the sample. We detected 183.3 GHz water masers in 5 out of 18 sources. IRAS 16293-2422 shows all three transitions, while Serpens FIRS 1 also displays the 321.2 GHz line. Despite excellent observing conditions, detection rates drop with increasing frequency, reflecting both intrinsic line weakness and variability. Notably, the brightest (sub)millimeter masers can reach flux densities comparable to the 22 GHz line. Comparisons of velocity profiles show that different transitions often trace distinct gas components. Water masers generally appear at earlier or comparable evolutionary stages than MM, suggesting no universal maser-based age sequence. Our results demonstrate the detectability of submillimeter water in low-mass SFRs, although their occurrence is sparse. Velocity overlap between some centimeter and millimeter components suggests partial spatial coincidence, but many features appear uniquely in one frequency regime, indicating that different transitions often trace distinct gas regions with varying physical conditions.

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arXiv2025

Protostellar Outflows at the EarliesT Stages (POETS) VI. Evidence of disk-wind in G11.92-0.61 MM1

Magnetohydrodynamic disk-winds play a key role in the formation of massive stars by providing the fine-tuning between accretion and ejection, where excess angular momentum is redirected away from the disk, allowing further mass growth. However, only a limited number of disk-wind sources have been detected to date. To better constrain the exact mechanism of this phenomenon, expanding the sample is critical. We performed an analysis of the disk-wind candidate G11.92-0.61 MM1 by estimating the physical parameters of the system and constraining the wind-launching mechanism. Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 observations were conducted in September 2021 with ALMA's longest baselines, which provided a synthesised beam of $\sim$30 mas. We obtained high-resolution images of the CH$_3$CN ($v_8=1$ and $v=0$), CH$_3$OH, SO$_2$, and SO molecular lines, and the 1.3 mm continuum. Our molecular data allowed us to refine the parameters of the disk-outflow system in MM1. The rotating disk is resolved into two regions with distinct kinematics: the inner region ($<$300 au) is traced by high-velocity emission of high-excitation CH$_3$CN lines and shows a Keplerian rotation; the outer region ($>$300 au), traced by mid-velocity CH$_3$CN emission, rotates in a sub-Keplerian regime. The central source is estimated to be $\sim$20 $M_{\odot}$, which is about half the mass estimated in previous lower-resolution studies. A strong collimated outflow is traced by SO and SO$_2$ emission up to $\sim$3400 au around MM1a. The SO and SO$_2$ emissions show a rotation-dominated velocity pattern, a constant specific angular momentum, and a Keplerian profile that suggests a magneto-centrifugal disk-wind origin with launching radii of $\sim$50-100 au. G11.92-0.61 MM1 appears to be one of the clearest cases of molecular line-traced disk-winds detected around massive protostars.

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arXiv2024

Broadband Multi-wavelength Properties of M87 during the 2018 EHT Campaign including a Very High Energy Flaring Episode

The nearby elliptical galaxy M87 contains one of the only two supermassive black holes whose emission surrounding the event horizon has been imaged by the Event Horizon Telescope (EHT). In 2018, more than two dozen multi-wavelength (MWL) facilities (from radio to gamma-ray energies) took part in the second M87 EHT campaign. The goal of this extensive MWL campaign was to better understand the physics of the accreting black hole M87*, the relationship between the inflow and inner jets, and the high-energy particle acceleration. Understanding the complex astrophysics is also a necessary first step towards performing further tests of general relativity. The MWL campaign took place in April 2018, overlapping with the EHT M87* observations. We present a new, contemporaneous spectral energy distribution (SED) ranging from radio to very high energy (VHE) gamma-rays, as well as details of the individual observations and light curves. We also conduct phenomenological modelling to investigate the basic source properties. We present the first VHE gamma-ray flare from M87 detected since 2010. The flux above 350 GeV has more than doubled within a period of about 36 hours. We find that the X-ray flux is enhanced by about a factor of two compared to 2017, while the radio and millimetre core fluxes are consistent between 2017 and 2018. We detect evidence for a monotonically increasing jet position angle that corresponds to variations in the bright spot of the EHT image. Our results show the value of continued MWL monitoring together with precision imaging for addressing the origins of high-energy particle acceleration. While we cannot currently pinpoint the precise location where such acceleration takes place, the new VHE gamma-ray flare already presents a challenge to simple one-zone leptonic emission model approaches, and emphasises the need for combined image and spectral modelling.

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arXiv2024

Ordered magnetic fields around the 3C 84 central black hole

3C84 is a nearby radio source with a complex total intensity structure, showing linear polarisation and spectral patterns. A detailed investigation of the central engine region necessitates the use of VLBI above the hitherto available maximum frequency of 86GHz. Using ultrahigh resolution VLBI observations at the highest available frequency of 228GHz, we aim to directly detect compact structures and understand the physical conditions in the compact region of 3C84. We used EHT 228GHz observations and, given the limited (u,v)-coverage, applied geometric model fitting to the data. We also employed quasi-simultaneously observed, multi-frequency VLBI data for the source in order to carry out a comprehensive analysis of the core structure. We report the detection of a highly ordered, strong magnetic field around the central, SMBH of 3C84. The brightness temperature analysis suggests that the system is in equipartition. We determined a turnover frequency of $\nu_m=(113\pm4)$GHz, a corresponding synchrotron self-absorbed magnetic field of $B_{SSA}=(2.9\pm1.6)$G, and an equipartition magnetic field of $B_{eq}=(5.2\pm0.6)$G. Three components are resolved with the highest fractional polarisation detected for this object ($m_\textrm{net}=(17.0\pm3.9)$%). The positions of the components are compatible with those seen in low-frequency VLBI observations since 2017-2018. We report a steeply negative slope of the spectrum at 228GHz. We used these findings to test models of jet formation, propagation, and Faraday rotation in 3C84. The findings of our investigation into different flow geometries and black hole spins support an advection-dominated accretion flow in a magnetically arrested state around a rapidly rotating supermassive black hole as a model of the jet-launching system in the core of 3C84. However, systematic uncertainties due to the limited (u,v)-coverage, however, cannot be ignored.

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arXiv2023

Metadata for the Flux Density Calibration of the April 2018 Event Horizon Telescope Data

The Event Horizon Telescope (EHT) observations carried out in 2018 April at 1.3 mm wavelengths included 9 stations in the array, comprising 7 single-dish telescopes and 2 phased arrays. The metadata package for the 2018 EHT observing campaign contains calibration tables required for the a-priori amplitude calibration of the 2018 April visibility data. This memo is the official documentation accompanying the release of the 2018 EHT metadata package, providing an overview of the contents of the package. We describe how telescope sensitivities, gain curves and other relevant parameters for each station in the EHT array were collected, processed, and validated to produce the calibration tables.

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arXiv2023

Monitoring of the polarized $H_{2}O$ maser emission around the massive protostars W75N(B)-VLA1 and W75N(B)-VLA2

Several radio sources have been detected in the HMSFR W75N(B), among them the massive YSOs VLA1 and VLA2 are of great interest. These are thought to be in different evolutionary stages. In particular, VLA1 is at the early stage of the photoionization and it is driving a thermal radio jet, while VLA2 is a thermal, collimated ionized wind surrounded by a dusty disk or envelope. In both sources 22 GHz water masers have been detected in the past. Those around VLA1 show a persistent distribution along the radio jet and those around VLA2 have instead traced the evolution from a non-collimated to a collimated outflow over a period of 20 years. By monitoring the polarized emission of the water masers around both VLA1 and VLA2 over a period of 6 years, we aim to determine whether the maser distributions show any variation over time and whether the magnetic field behaves accordingly. The EVN was used in full polarization and phase-reference mode to measure the absolute positions of the masers and to determine both the orientation and the strength of the magnetic field. We observed four epochs separated by two years from 2014 to 2020. We detected polarized emission from the water masers around both the YSOs in all the epochs. We find that the masers around VLA1 are tracing a nondissociative shock originating from the expansion of the thermal radio jet, while the masers around VLA2 are tracing an asymmetric expansion of the gas that is halted in the northeast where the gas likely encounters a very dense medium. We also found that the magnetic field inferred from the water masers in each epoch can be considered as a portion of a quasi-static magnetic field estimated in that location rather than in that time. This allowed us to study locally the morphology of the magnetic field around both YSOs in a larger area by considering the vectors estimated in all the epochs as a whole.

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arXiv2023

A Characterization of the ALMA Phasing System at 345 GHz

The development of the Atacama Large Millimeter/submillimeter Array (ALMA) phasing system (APS) has allowed ALMA to function as an extraordinarily sensitive station for very long baseline interferometry (VLBI) at frequencies of up to 230 GHz (~1.3 mm). Efforts are now underway to extend use of the APS to 345 GHz (~0.87 mm). Here we report a characterization of APS performance at 345 GHz based on a series of tests carried out between 2015-2021, including a successful global VLBI test campaign conducted in 2018 October in collaboration with the Event Horizon Telescope (EHT).

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arXiv2022

The sharp ALMA view of infall and outflow in the massive protocluster G31.41+0.31

Context. To better understand the formation of high-mass stars, it is fundamental to investigate how matter accretes onto young massive stars, how it is ejected, and how all this differs from the low-mass case. The massive protocluster G31.41+0.31 is the ideal target to study all these processes because observations at millimeter and centimeter wavelengths have resolved the emission of the Main core into at least four massive dust continuum sources, named A, B, C, and D, within 1" or 0.018 pc, and have identified signatures of infall and several outflows associated with the core. Aims. We study the interplay between infall and outflow in G31.41+0.31 by investigating at a spatial resolution of a few 100 au their properties and their possible impact on the core. Methods. We carried out molecular line observations of typical high-density tracers, such as CH3CN or H2CO, and shock and outflow tracers, such as SiO, with ALMA at 1.4 mm that achieved an angular resolution of 0.09" (340 au). Results. The observations have revealed inverse P-Cygni profiles in CH3CN and H2CO toward the four sources embedded in the Main core, suggesting that all of them are undergoing collapse. The infall rates, estimated from the red-shifted absorption are on the order of 1E-2 Msun/yr. The individual infall rates imply that the accretion timescale of the Main core is an order of magnitude smaller than its rotation timescale. This confirms that rotating toroids such as the G31 Main core are non-equilibrium, transient collapsing structures that need to be constantly replenished with fresh material from a large-scale reservoir. For sources B, C, and D, the infall could be accelerating inside the sources, while for source A, the presence of a second emission component complicates the interpretation. The SiO observations have revealed the presence of at least six outflows in the G31.41+0.31 star-forming region, ...

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arXiv2021

Broadband Multi-wavelength Properties of M87 during the 2017 Event Horizon Telescope Campaign

In 2017, the Event Horizon Telescope (EHT) Collaboration succeeded in capturing the first direct image of the center of the M87 galaxy. The asymmetric ring morphology and size are consistent with theoretical expectations for a weakly accreting supermassive black hole of mass approximately 6.5 x 10^9 M_solar. The EHTC also partnered with several international facilities in space and on the ground, to arrange an extensive, quasi-simultaneous multi-wavelength campaign. This Letter presents the results and analysis of this campaign, as well as the multi-wavelength data as a legacy data repository. We captured M87 in a historically low state, and the core flux dominates over HST-1 at high energies, making it possible to combine core flux constraints with the more spatially precise very long baseline interferometry data. We present the most complete simultaneous multi-wavelength spectrum of the active nucleus to date, and discuss the complexity and caveats of combining data from different spatial scales into one broadband spectrum. We apply two heuristic, isotropic leptonic single-zone models to provide insight into the basic source properties, but conclude that a structured jet is necessary to explain M87's spectrum. We can exclude that the simultaneous gamma-ray emission is produced via inverse Compton emission in the same region producing the EHT mm-band emission, and further conclude that the gamma-rays can only be produced in the inner jets (inward of HST-1) if there are strongly particle-dominated regions. Direct synchrotron emission from accelerated protons and secondaries cannot yet be excluded.

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arXiv2021

Fragmentation in the massive G31.41+0.31 protocluster

Context. ALMA observations at 1.4 mm and 0.2'' (750au) angular resolution of the Main core in the high-mass star forming region G31.41+0.31 have revealed a puzzling scenario: on the one hand, the continuum emission looks very homogeneous and the core appears to undergo solid-body rotation, suggesting a monolithic core stabilized by the magnetic field; on the other hand, rotation and infall speed up toward the core center, where two massive embedded free-free continuum sources have been detected, pointing to an unstable core having undergone fragmentation. Aims. To establish whether the Main core is indeed monolithic or its homogeneous appearance is due to a combination of large dust opacity and low angular resolution, we carried out millimeter observations at higher angular resolution and different wavelengths. Methods. We carried out ALMA observations at 1.4 mm and 3.5 mm that achieved angular resolutions of 0.1''(375 au) and 0.075'' (280 au), respectively. VLA observations at 7 mm and 1.3 cm at even higher angular resolution, 0.05'' (190 au) and 0.07'' (260 au), respectively, were also carried out to better study the nature of the free-free continuum sources detected in the core. Results. The millimeter continuum emission of the Main core has been clearly resolved into at least four sources, A, B, C, and D, within 1'', indicating that the core is not monolithic. The deconvolved radii of the dust emission of the sources, estimated at 3.5 mm, are 400-500au, their masses range from 15 to 26 Msun, and their number densities are several 1E9 cm-3. Sources A and B, located closer to the center of the core and separated by 750 au, are clearly associated with two free-free continuum sources, likely thermal radio jets, and are the brightest in the core. The spectral energy distribution of these two sources and their masses and sizes are similar and suggest a common origin.

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arXiv2020

On the pumping of the CS($v=0$) masers in W51 e2e

We present the results of numerically solving the rate equations for the first 31 rotational states of CS in the ground vibrational state to determine the conditions under which the J=1-0, J=2-1 and J=3-2 transitions are inverted to produce maser emission. The essence of our results is that the CS($v=0$) masers are collisionally pumped and that, depending on the spectral energy distribution, dust emission can suppress the masers. Apart from the J=1-0 and J=2-1 masers the calculations also show that the J=3-2 transition can be inverted to produce maser emission. It is found that beaming is necessary to explain the observed brightness temperatures of the recently discovered CS masers in W51 e2e. The model calculations suggest that a CS abundance of a few times $10^{-5}$ and CS($v=0$) column densities of the order $10^{16}\,\mathrm{cm^{-2}}$ are required for these masers. The rarity of the CS masers in high mass star forming regions might be the result of a required high CS abundance as well as due to attenuation of the maser emission inside as well as outside of the hot core.

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arXiv2020

Characterizing the radio continuum nature of sources in the massive star-forming region W75N (B)

The massive star-forming region W75N~(B) is thought to host a cluster of massive protostars (VLA~1, VLA~2, and VLA~3) undergoing different evolutionary stages. In this work, we present radio continuum data with the highest sensitivity and angular resolution obtained to date in this region, using the VLA-A and covering a wide range of frequencies (4-48~GHz), which allowed us to study the morphology and the nature of the emission of the different radio continuum sources. We also performed complementary studies with multi-epoch VLA data and ALMA archive data at 1.3 mm wavelength. We find that VLA~1 is driving a thermal radio jet at scales of $\approx$0.1 arcsec ($\approx$130 au), but also shows signs of an incipient hyper-compact HII region at scales of $\lesssim$ 1 arcsec ($\lesssim$ 1300~au). VLA~3 is also driving a thermal radio jet at scales of a few tenths of arcsec (few hundred of au). We conclude that this jet is shock-exciting the radio continuum sources Bc and VLA~4 (obscured HH objects), which show proper motions moving outward from VLA~3 at velocities of $\approx$112--118~km/s. We have also detected three new weak radio continuum sources, two of them associated with millimeter continuum cores observed with ALMA, suggesting that these two sources are also embedded YSOs in this massive star-forming region.

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arXiv2020

SYMBA: An end-to-end VLBI synthetic data generation pipeline

Realistic synthetic observations of theoretical source models are essential for our understanding of real observational data. In using synthetic data, one can verify the extent to which source parameters can be recovered and evaluate how various data corruption effects can be calibrated. These studies are important when proposing observations of new sources, in the characterization of the capabilities of new or upgraded instruments, and when verifying model-based theoretical predictions in a comparison with observational data. We present the SYnthetic Measurement creator for long Baseline Arrays (SYMBA), a novel synthetic data generation pipeline for Very Long Baseline Interferometry (VLBI) observations. SYMBA takes into account several realistic atmospheric, instrumental, and calibration effects. We used SYMBA to create synthetic observations for the Event Horizon Telescope (EHT), a mm VLBI array, which has recently captured the first image of a black hole shadow. After testing SYMBA with simple source and corruption models, we study the importance of including all corruption and calibration effects. Based on two example general relativistic magnetohydrodynamics (GRMHD) model images of M87, we performed case studies to assess the attainable image quality with the current and future EHT array for different weather conditions. The results show that the effects of atmospheric and instrumental corruptions on the measured visibilities are significant. Despite these effects, we demonstrate how the overall structure of the input models can be recovered robustly after performing calibration steps. With the planned addition of new stations to the EHT array, images could be reconstructed with higher angular resolution and dynamic range. In our case study, these improvements allowed for a distinction between a thermal and a non-thermal GRMHD model based on salient features in reconstructed images.

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arXiv2019

Substructures in the Keplerian disc around the O-type (proto)star G17.64+0.16

We present the highest angular resolution (20x15mas - 44x33au) Atacama Large Millimeter/sub-millimeter Array (ALMA) observations currently possible of the proto-O-star G17.64+0.16 in Band 6. The Cycle 5 observations with baselines out to 16km probes scales <50au and reveal the rotating disc around G17.64+0.16, a massive forming O-type star. The disc has a ring-like enhancement in the dust emission, especially visible as arc structures to the north and south. The Keplerian kinematics are most prominently seen in the vibrationally excited water line, H2O (Eu=3461.9K). The mass of the central source found by modelling the Keplerian rotation is consistent with 45+/-10Mo. The H30alpha (231.9GHz) radio-recombination line and the SiO (5-4) molecular line were detected at up to the 10 sigma$ level. The estimated disc mass is 0.6-2.6Mo under the optically thin assumption. Analysis of the Toomre Q parameter, in the optically thin regime, indicates that the disc stability is highly dependent on temperature. The disc currently appears stable for temperatures >150K, this does not preclude that the substructures formed earlier through disc fragmentation.

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arXiv2019

rPICARD: A CASA-based Calibration Pipeline for VLBI Data. Calibration and imaging of 7 mm VLBA observations of the AGN jet in M87

(Abridged) The CASA software suite, can now reduce very long baseline interferometry (VLBI) data with the recent addition of a fringe fitter. Here, we present the Radboud PIpeline for the Calibration of high Angular Resolution Data (rPICARD), which is an open-source VLBI calibration and imaging pipeline built on top of the CASA framework. The pipeline is capable of reducing data from different VLBI arrays. It can be run non-interactively after only a few non-default input parameters are set and delivers high-quality calibrated data. CPU scalability based on a message-passing interface (MPI) implementation ensures that large bandwidth data from future arrays can be processed within reasonable computing times. Phase calibration is done with a Schwab-Cotton fringe fit algorithm. For the calibration of residual atmospheric effects, optimal solution intervals are determined based on the signal-to-noise ratio (S/N) of the data for each scan. Different solution intervals can be set for different antennas in the same scan to increase the number of detections in the low S/N regime. These novel techniques allow rPICARD to calibrate data from different arrays, including high-frequency and low-sensitivity arrays. The amplitude calibration is based on standard telescope metadata, and a robust algorithm can solve for atmospheric opacity attenuation in the high-frequency regime. Standard CASA tasks are used for CLEAN imaging and self-calibration. In this work we demonstrate the capabilities of rPICARD by calibrating and imaging 7 mm VLBA data of the central radio source in the M87 galaxy. The reconstructed jet image reveals a complex collimation profile and edge-brightened structure. A potential counter-jet is detected that has 10 % of the brightness of the approaching jet. This constrains jet speeds close to the radio core to about half the speed of light for small inclination angles.

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arXiv2019

Protostellar Outflows at the EarliesT Stages (POETS). II. A possible radio synchrotron jet associated with the EGO G035.02+0.35

Centimeter continuum observations of protostellar jets have revealed the presence of knots of shocked gas where the flux density decreases with frequency. This spectrum is characteristic of nonthermal synchrotron radiation and implies the presence of both magnetic fields and relativistic electrons in protostellar jets. Here, we report on one of the few detections of nonthermal jet driven by a young massive star in the star-forming region G035.02$+$0.35. We made use of the NSF's Karl G. Jansky Very Large Array (VLA) to observe this region at C, Ku, and K bands with the A- and B-array configurations, and obtained sensitive radio continuum maps down to a rms of 10 $\mu$Jy beam$^{-1}$. These observations allow for a detailed spectral index analysis of the radio continuum emission in the region, which we interpret as a protostellar jet with a number of knots aligned with extended 4.5 $\mu$m emission. Two knots clearly emit nonthermal radiation and are found at similar distances, of approximately 10,000 au, each side of the central young star, from which they expand at velocities of hundreds km s$^{-1}$. We estimate both the mechanical force and the magnetic field associated with the radio jet, and infer a lower limit of $0.4\times10^{-4} $M$_{\odot}$ yr$^{-1}$ km s$^{-1}$ and values in the range $0.7-1.3 $mG, respectively.

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