arXiv subjects

A. Raga

Publications and source records attributed to A. Raga.

At least 19 recordsLinked to original sources

arXiv2021

Huffing, and puffing, and blowing your house in: Strong stellar winds interaction with a super massive black hole

We present analytic and numerical models of a cluster wind flow resulting from the interaction of stellar winds of massive stars, with a super massive black hole (SMBH). We consider the motion of the stars as well as the gravitational force of the SMBH. In the numerical simulations we consider two cases: the first one with the stars is in circular orbits, and the second one with the stars in eccentric orbits around the SMBH. We found that after the system reaches an equilibrium, the circular and elliptical cases are very similar. We found a very good agreement between the analytical and numerical results, not only from our numerical simulations but also from other high resolution numerical calculations. The analytical models are very interesting, since the properties of such complex systems involving strong winds and a massive compact object, can be rapidly inferred without the need of a numerical calculation.

astro-ph.GA↗

arXiv2018

Interaction between a pulsating jet and a surrounding disk wind. A hydrodynamical perspective

The molecular richness of fast protostellar jets within 20-100 au of their source, despite strong ultraviolet irradiation, remains a challenge for the models investigated so far. We aim to investigate the effect of interaction between a time-variable jet and a surrounding steady disk wind, to assess the possibility of jet chemical enrichement by the wind, and the characteristic signatures of such a configuration. We have constructed an analytic model of a jet bow shock driven into a surrounding slower disk wind in the thin shell approximation. The refilling of the post bow shock cavity from below by the disk wind is also studied. An extension of the model to the case of two or more successive internal working surfaces (IWS) is made. We then compared this analytic model with numerical simulations with and without a surrounding disk wind. We find that at early times (of order the variability period), jet bow shocks travel in refilled pristine disk-wind material, before interacting with the cocoon of older bow shocks. This opens the possibility of bow shock chemical enrichment (if the disk wind is molecular and dusty) and of probing the unperturbed disk wind structure near the jet base. Several distinctive signatures of the presence of a surrounding disk wind are identified, in the bow shock morphology and kinematics. Numerical simulations validate our analytical approach and further show that at large scale, the passage of many jet IWS inside a disk wind produces a stationary V-shaped cavity, closing down onto the axis at a finite distance from the source.

astro-ph.SR↗

arXiv2016

Bow shock nebulae of hot massive stars in a magnetized medium

A significant fraction of OB-type, main-sequence massive stars are classified as runaway and move supersonically through the interstellar medium (ISM). Their strong stellar winds interact with their surroundings where the typical strength of the local ISM magnetic field is about 3.5-7 micro-G, which can result in the formation of bow shock nebulae. We investigate the effects of such magnetic fields, aligned with the motion of the flow, on the formation and emission properties of these circumstellar structures. Our axisymmetric, magneto-hydrodynamical simulations with optically-thin radiative cooling, heating and anisotropic thermal conduction show that the presence of the background ISM magnetic field affects the projected optical emission our bow shocks at Ha and [OIII] lambda 5007 which become fainter by about 1-2 orders of magnitude, respectively. Radiative transfer calculations against dust opacity indicate that the magnetic field slightly diminishes their projected infrared emission and that our bow shocks emit brightly at 60 micron. This may explain why the bow shocks generated by ionizing runaway massive stars are often difficult to identify. Finally, we discuss our results in the context of the bow shock of Zeta Ophiuchi and we support the interpretation of its imperfect morphology as an evidence of the presence of an ISM magnetic field not aligned with the motion of its driving star.

astro-ph.SR↗

arXiv2014

The Emerging Planetary Nebula CRL 618 and its Unsettled Central Star(s)

We report deep long-slit emission-line spectra, the line flux ratios, and Doppler profile shapes of various bright optical lines. The low-ionization lines (primarily [N I], [O I], [S II], and [N II]) originate in shocked knots, as reported by many previous observers. Dust-scattered lines of higher ionization are seen throughout the lobes but do not peak in the knots. Our analysis of these line profiles and the readily discernible stellar continuum shows that (1) the central star is an active symbiotic (whose spectrum resembles the central stars of highly bipolar and young PNe such as M2-9 and Hen2-437) whose compact companion shows a WC8-type spectrum, (2) extended nebular lines of [O III] and He I originate in the heavily obscured nuclear H II region, and (3) the Balmer lines observed throughout the lobes are dominated by reflected Hα emission from the symbiotic star. Comparing our line ratios with those observed historically shows that (1) the [O III]/H\b{eta} and He I/H\b{eta} ratios have been steadily rising by large amounts throughout the nebula, (2) the Hα/H\b{eta} ratio is steadily decreasing while Hγ/H\b{eta} remains nearly constant, and (3) the low-ionization line ratios formed in the shocked knots have been in decline in different ways at various locations. We show that the first two of these results might be expected if the symbiotic central star has been active and if its bright Hα line has faded significantly in the past 20 years.

astro-ph.SR↗

arXiv2014

Jets and Outflows From Star to Cloud: Observations Confront Theory

In this review we focus on the role jets and outflows play in the star and planet formation process. Our essential question can be posed as follows: are jets/outflows merely an epiphenomenon associated with star formation or do they play an important role in mediating the physics of assembling stars both individually and globally? We address this question by reviewing the current state of observations and their key points of contact with theory. Our review of jet/outflow phenomena is organized into three length-scale domains: Source and Disk Scales ($0.1-10^2$ au) where the connection with protostellar and disk evolution theories is paramount; Envelope Scales ($10^2-10^5$ au) where the chemistry and propagation shed further light on the jet launching process, its variability and its impact on the infalling envelope; Parent Cloud Scales ($10^5-10^6$ au) where global momentum injection into cluster/cloud environments become relevant. Issues of feedback are of particular importance on the smallest scales where planet formation regions in a disk may be impacted by the presence of disk winds, irradiation by jet shocks or shielding by the winds. Feedback on envelope scales may determine the final stellar mass (core-to-star efficiency) and envelope dissipation. Feedback also plays an important role on the larger scales with outflows contributing to turbulent support within clusters including alteration of cluster star formation efficiencies (feedback on larger scales currently appears unlikely). A particularly novel dimension of our review is that we consider results on jet dynamics from the emerging field of High Energy Density Laboratory Astrophysics (HEDLA). HEDLA is now providing direct insights into the 3-D dynamics of fully magnetized, hypersonic, radiative outflows.

astro-ph.SR↗

arXiv2013

On the sensitivity of extrasolar mass-loss rate ranges: HD 209458b a case study

We present a 3D hydrodynamic study of the effects that different stellar wind conditions and planetary wind structures have on the calculated Ly-$α$ absorptions produced during the transit of HD 209458b. Considering a range of stellar wind speeds $\sim$[350-800] km s$^{-1}$, coronal temperature $\sim$[3-7] $\times10^{6}$ K and two values of the polytropic index $Γ$ $\sim$[1.01-1.13], while keeping fixed the stellar mass loss rate, we found a that a $\dot M_p$ range between $\sim$[3-5] $\times 10^{10}$g s$^{-1}$ give account for the observational absorption in Ly-$α$ measured for the planetary system. Also, several models with anisotropic evaporation profiles for the planetary escaping atmosphere were carried out, showing that both, the escape through polar regions and through the night side yields larger absorptions than an isotropic planetary wind.

astro-ph.SR↗

arXiv2013

Multi-epoch VLBA H$_2$O maser observations toward the massive YSOs AFGL 2591 VLA 2 and VLA 3

We present multi-epoch Very Long Baseline Array (VLBA) H$_2$O maser observations toward the massive young stellar objects (YSOs) VLA 2 and VLA 3 in the star-forming region AFGL 2591. Through these observations, we have extended the study of the evolution of the masers towards these objects up to a time span of $\sim$ 10 yrs, measuring their radial velocities and proper motions. The H$_2$O masers in VLA 3, the most massive YSO in AFGL 2591 ($\sim$ 30--40~M$_{\odot}$), are grouped within projected distances of $\lesssim$ 40 mas ($\lesssim$ 130 AU) from VLA 3. In contrast to other H$_2$O masers in AFGL 2591, the masers associated with VLA 3 are significantly blueshifted (up to $\sim$ 30 km s$^{-1}$) with respect to the velocity of the ambient molecular cloud. We find that the H$_2$O maser cluster as a whole, has moved westwards of VLA~3 between the 2001 and 2009 observations, with a proper motion of $\sim$ 1.2 mas yr$^{-1}$ ($\sim$ 20 km s$^{-1}$). We conclude that these masers are tracing blueshifted outflowing material, shock excited at the inner parts of a cavity seen previously in ammonia molecular lines and infrared images, and proposed to be evacuated by the outflow associated with the massive VLA 3 source. The masers in the region of VLA 2 are located at projected distances of $\sim$ 0.7$''$ ($\sim$ 2300 AU) north from this source, with their kinematics suggesting that they are excited by a YSO other than VLA 2. This driving source has not yet been identified.

astro-ph.GA↗

arXiv2012

Formation and evolution of the water maser outflow event in AFGL 2591 VLA 3-N

In this paper we analyze multi-epoch Very Long Baseline Interferometry (VLBI) water maser observations carried out with the Very Long Baseline Array (VLBA) toward the high-mass star-forming region AFGL 2591. We detected maser emission associated with the radio continuum sources VLA 2 and VLA 3. In addition, a water maser cluster, VLA 3-N, was detected ~ 0.5" north of VLA 3. We concentrate the discussion of this paper on the spatio-kinematical distribution of the water masers towards VLA 3-N. The water maser emission toward the region VLA 3-N shows two bow shock-like structures, Northern and Southern, separated from each other by ~ 100 mas (~ 330 AU). The spatial distribution and kinematics of the water masers in this cluster have persisted over a time span of seven years. The Northern bow shock has a somewhat irregular morphology, while the Southern one has a remarkably smooth morphology. We measured the proper motions of 33 water maser features, which have an average proper motion velocity of ~ 1.3 mas/yr (~ 20 km/s). The morphology and the proper motions of this cluster of water masers show systematic expanding motions that could imply one or two different centers of star formation activity. We made a detailed model for the Southern structure, proposing two different kinematic models to explain the 3-dimensional spatio-kinematical distribution of the water masers: (1) a static central source driving the two bow-shock structures; (2) two independent driving sources, one of them exciting the Northern bow-shock structure, and the other one, a young runaway star moving in the local molecular medium exciting and molding the remarkably smoother Southern bow-shock structure. Future observations will be necessary to discriminate between the two scenarios, in particular by identifying the still unseen driving source(s).

astro-ph.SR↗

arXiv2010

A wide-angle outflow with the simultaneous presence of a high-velocity jet in the high-mass Cepheus A HW2 system

We present five epochs of VLBI water maser observations around the massive protostar Cepheus A HW2 with 0.4 mas (0.3 AU) resolution. The main goal of these observations was to follow the evolution of the remarkable water maser linear/arcuate structures found in earlier VLBI observations. Comparing the data of our new epochs of observation with those observed five years before, we find that at "large" scales of > 1" (700 AU) the main regions of maser emission persist, implying that both the surrounding medium and the exciting sources of the masers have been relatively stable during that time span. However, at smaller scales of < 0.1" (70 AU) we see large changes in the maser structures, particularly in the expanding arcuate structures R4 and R5. R4 traces a nearly elliptical patchy ring of ~ 70 mas size (50 AU) with expanding motions of ~ 5 mas/yr (15 km/s). This structure is probably driven by the wind of a still unidentified YSO located at the centre of the ring (~ 0.18" south of HW2). On the other hand, the R5 expanding bubble structure (driven by the wind of a previously identified YSO located ~ 0.6" south of HW2) is currently dissipating in the circumstellar medium and losing its previous degree of symmetry, indicating a very short-lived event. In addition, our results reveal, at scales of ~ 1" (700 AU), the simultaneous presence of a relatively slow (~ 10-70 km/s) wide-angle outflow (opening angle of ~ 102 deg, traced by the masers, and the fast (~ 500~km/s) highly collimated radio jet associated with HW2 (opening angle of ~ 18 deg, previously observed with the VLA. This simultaneous presence of a wide-angle outflow and a highly collimated jet associated with a massive protostar is similar to what is found in some low-mass YSOs. The implications of these results in the study of the formation of high-mass stars are discussed.

astro-ph.SR↗

arXiv2009

An upper limit on the mass of the black hole in Ursa Minor dwarf galaxy

The well-established correlations between the mass of massive black holes (BHs) in the nuclei of most studied galaxies and various global properties of their hosting galaxy lend support to the idea that dwarf galaxies and globular clusters could also host a BH in their centers. Direct kinematic detection of BHs in dwarf spheroidal (dSph) galaxies are seriously hindered by the small number of stars inside the gravitational influence region of the BH. The aim of this Letter is to establish an upper dynamical limit on the mass of the putative BH in the Ursa Minor (UMi) dSph galaxy. We present direct N-body simulations of the tidal disruption of the dynamical fossil observed in UMi, with and without a massive BH. We find that the observed substructure is incompatible with the presence of a massive BH of (2-3)x10^4 Msun within the core of UMi. These limits are consistent with the extrapolation of the M_{BH}-sigma relation to the M_{BH}<10^6 Msun regime. We also show that the BH may be off-center with respect to the center of symmetry of the whole galaxy.

astro-ph.CO↗

arXiv2008

Supernova Explosions and the Triggering of Galactic Fountains and Outflows

We review here the effects of supernovae (SNe) explosions on the environment of star-forming galaxies. Randomly distributed, clustered SNe explosions cause the formation of hot superbubbles that drive either galactic fountains or supersonic winds out of the galactic disk. In a galactic fountain, the ejected gas is re-captured by the gravitational potential and falls back onto the disk. From 3D non-equilibrium radiative cooling hydrodynamical simulations of these fountains, we find that they may reach altitudes smaller than 5 kpc in the halo and hence explain the formation of the so-called intermediate-velocity-clouds (IVCs). On the other hand, the high-velocity-clouds (HVCs) that are observed at higher altitudes (of up to 12 kpc) require another mechanism to explain their production. We argue that they could be formed either by the capture of gas from the intergalactic medium and/or by the action of magnetic fields that are carried out to the halo with the gas in the fountains. Due to angular momentum losses (of 10-15%) to the halo, we find that the fountain material falls back to smaller radii and is not largely spread over the galactic disk, as previously expected. This result is consistent with the metal distribution derived from recent chemical models of the galaxy. We also find that after about 150 Myr, the gas circulation between the halo and the disk in the fountains reaches a steady state regime (abridged).

astro-ph↗

arXiv2008

Multiple and Precessing Collimated Outflows in the Planetary Nebula IC 4634

With its remarkable double-S shape, IC 4634 is an archetype of point-symmetric planetary nebulae (PN). In this paper, we present a detailed study of this PN using archival HST WFPC2 and ground-based narrow-band images to investigate its morphology, and long-slit spectroscopic observations to determine its kinematics and to derive its physical conditions and excitation. The data reveal new structural components, including a distant string of knots distributed along an arc-like feature 40"-60" from the center of the nebula, a skin of enhanced [O III]/H-alpha ratio enveloping the inner shell and the double-S feature, and a triple-shell structure. The spatio-kinematical study also finds an equatorial component of the main nebula that is kinematically independent from the bright inner S-shaped arc. We have investigated in detail the bow shock-like features in IC 4634 and found that their morphological, kinematical and emission properties are consistent with the interaction of a collimated outflow with surrounding material. Indeed, the morphology and kinematics of some of these features can be interpreted using a 3D numerical simulation of a collimated outflow precessing at a moderate, time-dependent velocity. Apparently, IC 4634 has experienced several episodes of point-symmetric ejections oriented at different directions with the outer S-shaped feature being related to an earlier point-symmetric ejection and the outermost arc-like string of knots being the relic of an even much earlier point-symmetric ejection. There is tantalizing evidence that the action of these collimated outflows has also taken part in the shaping of the innermost shell and inner S-shaped arc of IC 4634.

astro-ph↗

arXiv2008

Galactic Outflows and the pollution of the Galactic Environment by Supernovae

We here explore the effects of the SN explosions into the environment of star-forming galaxies like the Milky Way. Successive randomly distributed and clustered SNe explosions cause the formation of hot superbubbles that drive either fountains or galactic winds above the galactic disk, depending on the amount and concentration of energy that is injected by the SNe. In a galactic fountain, the ejected gas is re-captured by the gravitational potential and falls back onto the disk. From 3D nonequilibrium radiative cooling hydrodynamical simulations of these fountains, we find that they may reach altitudes up to about 5 kpc in the halo and thus allow for the formation of the so called intermediate-velocity-clouds (IVCs) which are often observed in the halos of disk galaxies. The high-velocity-clouds that are also observed but at higher altitudes (of up to 12 kpc) require another mechanism to explain their production. We argue that they could be formed either by the capture of gas from the intergalactic medium and/or by the action of magnetic fields that are carried to the halo with the gas in the fountains. Due to angular momentum losses to the halo, we find that the fountain material falls back to smaller radii and is not largely spread over the galactic disk. Instead, the SNe ejecta fall nearby the region where the fountain was produced, a result which is consistent with recent chemical models of the galaxy. The fall back material leads to the formation of new generations of molecular clouds and to supersonic turbulence feedback in the disk.

astro-ph↗

arXiv2007

Erratum to Star formation triggered by SN explosions: an application to the stellar association of $β$Pictoris

In the present study, considering the physical conditions that are relevant interactions between supernova remnants (SNRs) and dense molecular clouds for triggering star formation we have built a diagram of SNR radius versus cloud density in which the constraints above delineate a shaded zone where star formation is allowed. We have also performed fully 3-D radiatively cooling numerical simulations of the impact between SNRs and clouds under different initial conditions in order to follow the initial steps of these interactions. We determine the conditions that may lead either to cloud collapse and star formation or to complete cloud destruction and find that the numerical results are consistent with those of the SNR-cloud density diagram. Finally, we have applied the results above to the $β-$ Pictoris stellar association which is composed of low mass Post-T Tauri stars with an age of 11 Myr. It has been recently suggested that its formation could have been triggered by the shock wave produced by a SN explosion localized at a distance of about 62 pc that may have occurred either in the Lower Centaurus Crux (LCC) or in the Upper Centaurus Lupus (UCL) which are both nearby older subgroups of that association (Ortega and co-workers).Using the results of the analysis above we have shown that the suggested origin for the young association at the proposed distance is plausible only for a very restricted range of initial conditions for the parent molecular cloud, i.e., a cloud with a radius of the order of 10 pc, a density of the order of 10$-$20 cm$^{-3}$, and a temperature of the order of 10$-$100 K }.

astro-ph↗

arXiv2007

A precessing jet model for the PN K 3-35: simulated radio-continuum emission

The bipolar morphology of the planetary nebula (PN) K 3-35 observed in radio-continuum images was modelled with 3D hydrodynamic simulations with the adaptive grid code yguazu-a. We find that the observed morphology of this PN can be reproduced considering a precessing jet evolving in a dense AGB circumstellar medium, given by a mass loss rate \dot{M}_{csm}=5x10^{-5}M_{\odot}/yr and a terminal velocity v_{w}=10 km/s. Synthetic thermal radio-continuum maps were generated from numerical results for several frequencies. Comparing the maps and the total fluxes obtained from the simulations with the observational results, we find that a model of precessing dense jets, where each jet injects material into the surrounding CSM at a rate \dot{M}_j=2.8x10^{-4} {M_{\odot}/yr (equivalent to a density of 8x10^{4} {cm}^{-3}, a velocity of 1500 km/s, a precession period of 100 yr, and a semi-aperture precession angle of 20 degrees agrees well with the observations.

astro-ph↗

arXiv2006

Star formation triggered by SN explosions: an application to the stellar association of $β$ Pictoris

In the present study, considering the physical conditions that are relevant in interactions between supernova remnants (SNRs) and dense molecular clouds for triggering star formation we have built a diagram of SNR radius versus cloud density in which the constraints above delineate a shaded zone where star formation is allowed. We have also performed fully 3-D radiatively cooling numerical simulations of the impact between SNRs and clouds under different initial conditions in order to follow the initial steps of these interactions. We determine the conditions that may lead either to cloud collapse and star formation or to complete cloud destruction and find that the numerical results are consistent with those of the SNR-cloud density diagram. Finally, we have applied the results above to the $β-$Pictoris stellar association which is composed of low mass Post-T Tauri stars with an age of 11 Myr. It has been recently suggested that its formation could have been triggered by the shock wave produced by a SN explosion localized at a distance of about 62 pc that may have occurred either in the Lower Centaurus Crux (LCC) or in the Upper Centaurus Lupus (UCL) which are both nearby older subgroups of that association (Ortega and co-workers). Using the results of the analysis above we have shown that the suggested origin for the young association at the proposed distance is plausible only for a very restricted range of initial conditions for the parent molecular cloud, i.e., a cloud with a radius of the order of 10 pc and density of the order of 20 cm$^{-3}$ and a temperature of the order of 50$-$100 K.

astro-ph↗

arXiv2005

Multidimensional Hydrodynamical Simulations of radiative cooling SNRs-clouds interactions: an application to Starburst Environments

In this work we are interested to study the by-products of SNR-clouds in a starburst (SB) system. These interactions can have an important role in the recycling of matter from the clouds to the ISM and vice-versa. In the present work, we have focused our attention on the global effects of the interactions between clouds and SN shock waves in the ISM of SB environments, and performed 3-D radiative cooling hydrodynamical simulations with the adaptive YGUAZU grid code. We have also considered the effects of the photo-evaporation due to the presence of a high number of UV photons from hot stars and supernovae (SNe). The results have shown that, in the presence of radiative cooling, instead of an efficient gas mixing with the diffuse ISM, the interactions cause the fragmentation of the clouds into smaller ones. The results have also revealed that the SNR-clouds interactions are less efficient at producing substantial mass loss from the clouds to the diffuse ISM than mechanisms such as the photo-evaporation caused by the UV flux from the hot stars.

astro-ph↗

arXiv2004

Hydrodynamical interaction between an accretion flow and a stellar wind

Molecular clouds in the interstellar medium suffer gravitational instabilities that lead to the formation of one or multiple stars. A recently formed star inside a cold cloud communicates its gravitational force to the surrounding environment and soon an accretion flow falling into the star develops. After their formation, all stars soon eject a wind of gas that interacts with the external accretion flow. This interaction produces a shock wave that evolves with time. The work presented in this article formulates a simple prescription for the evolution of this interaction. With the aid of this model we construct a few radio continuum maps of the source.

astro-ph↗