arXiv subjects

A. C. Raga

Publications and source records attributed to A. C. Raga.

At least 37 records · Page 2Linked to original sources

arXiv2016

The time evolution of HH~2 from four epochs of HST images

We have analyzed four epochs of H$α$ and [S~II] HST images of the HH~1/2 outflow (covering a time interval from 1994 to 2014) to determine proper motions and emission line fluxes of the knots of HH~2. We find that our new proper motions agree surprisingly well with the motions measured by Herbig \& Jones (1981), although there is partial evidence for a slight deceleration of the motion of the HH~2 knots from 1945 to 2014. We also measure the time-variability of the H$α$ intensities and the [S~II]/H$α$ line ratios, and find that knots H and A have the largest intensity variabilities (in $1994\to 2014$). Knot H (which now dominates the HH~2 emission) has strengthened substantially, while keeping an approximately constant [S~II]/H$α$ ratio. Knot A has dramatically faded, and at the same time has had a substantial increase in its [S~II]/H$α$ ratio. Possible interpretations of these results are discussed.

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arXiv2016

Observations and 3D Hydrodynamical models of planetary nebulae with Wolf Rayet type central stars

We present high-resolution, long-slit spectroscopic observations of two planetary nebulae with [WC] central stars located near the galactic bulge, M 1-32 and M 3-15. The observations were obtained with the 2.1-m telescope at the Observatorio Astronómico Nacional, San Pedro Mártir. M 1-32 shows wide wings on the base of its emission lines and M 3-15 has two very faint high-velocity knots. In order to model both PNe we built a three-dimensional model consisting of a jet interacting with an equatorially concentrated slow wind, emulating the presence of a dense torus, using the Yguazú hydrodynamical code. From our hydrodynamical models, we obtained position-velocity (PV) diagrams in the [N II]$λ$6583 line for comparison with the observations. We find that the spectral characteristics of M 1-32 and M 3-15 can be explained with the same physical model -a jet moving inside an AGB wind- using different parameters (physical conditions and position angles of the jet). In agreement with our model and observations, these objects contain a dense torus seeing pole-on and a bipolar jet escaping thorough the poles. Then we propose to classify this kind of objects as spectroscopic bipolar nebulae, although they have been classified morphologically as compact, round, or elliptical nebulae or with "close collimated lobes".

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arXiv2016

The time evolution of HH 1 from four epochs of HST images

We present an analysis of four epochs of H$α$ and [S II] $λλ$ 6716/6731 HST images of HH 1. For determining proper motions we explore a new method based on analysis of spatially degraded images obtained convolving the images with wavelet functions of chosen widths. With this procedure we are able to generate maps of proper motion velocities along and across the outflow axis, as well as (angularly integrated) proper motion velocity distributions. From the four available epochs, we find the time evolution of the velocities, intensities and spatial distribution of the line emission. We find that over the last two decades HH 1 shows a clear acceleration. Also, the H$α$ and [S II] intensities have first dropped, and then recovered in the more recent (2014) images. Finally, we show a comparison between the two available HST epochs of [O III] $λ$ 5007 (1994 and 2014), in which we see a clear drop in the value of the [O III]/H$α$ ratio.

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arXiv2016

Photo-ionization of planetary winds: case study HD209458b

Close-in hot Jupiters are exposed to a tremendous photon flux that ionizes the neutral escaping material from the planet leaving an observable imprint that makes them an interesting laboratory for testing theoretical models. In this work we present 3D hydrodynamic simulations with radiation transfer calculations of a close-in exoplanet in a blow-off state. We calculate the Ly-$α$ absorption and compare it with observations of HD 209458b an previous simplified model results.Our results show that the hydrodynamic interaction together with a proper calculation of the photoionization proccess are able to reproduce the main features of the observed Ly-$α$ absorption, in particular at the blue-shifted wings of the line. We found that the ionizing stellar flux produce an almost linear effect on the amount of absorption in the wake. Varying the planetary mass loss rate and the radiation flux, we were able to reproduce the $10\%$ absorption observed at $-100~\mathrm{km~s^{-1}}$.

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arXiv2015

StarBench: The D-type expansion of an HII region

StarBench is a project focused on benchmarking and validating different star-formation and stellar feedback codes. In this first StarBench paper we perform a comparison study of the D-type expansion of an HII region. The aim of this work is to understand the differences observed between the twelve participating numerical codes against the various analytical expressions examining the D-type phase of HII region expansion. To do this, we propose two well-defined tests which are tackled by 1D and 3D grid- and SPH- based codes. The first test examines the `early phase' D-type scenario during which the mechanical pressure driving the expansion is significantly larger than the thermal pressure of the neutral medium. The second test examines the `late phase' D-type scenario during which the system relaxes to pressure equilibrium with the external medium. Although they are mutually in excellent agreement, all twelve participating codes follow a modified expansion law that deviates significantly from the classical Spitzer solution in both scenarios. We present a semi-empirical formula combining the two different solutions appropriate to both early and late phases that agrees with high-resolution simulations to $\lesssim2\%$. This formula provides a much better benchmark solution for code validation than the Spitzer solution. The present comparison has validated the participating codes and through this project we provide a dataset for calibrating the treatment of ionizing radiation hydrodynamics codes.

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arXiv2015

Gemini-IFU spectroscopy of HH 111

We present new optical observations of the HH 111 Herbig-Haro jet using the Gemini Multi Object Spectrograph in its Integral Field Unit mode. Eight fields of 5" x 3.5" have been positioned along and across the HH 111 jet, covering the spatial region from knot E to L in HH 111 (namely, knots E, F, G, H, J, K and L). We present images and velocity channel maps for the [O I] 6300+6360, Halpha, [N II] 6548+6583 and [S II] 6716+6730 lines, as well as for the [S II]6716/6730 line ratio. We find that the HH 111 jet has an inner region with lower excitation and higher radial velocity, surrounded by a broader region of higher excitation and lower radial velocity. Also, we find higher electron densities at lower radial velocities. These results imply that the HH 111 jet has a fast, axial region with lower velocity shocks surrounded by a lower velocity sheath with higher velocity shocks.

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arXiv2014

Collisionally excited filaments in HST H$α$ and H$β$ images of HH~1/2

We present new H$α$ and H$β$ images of the HH~1/2 system, and we find that the H$α$/H$β$ ratio has high values in ridges along the leading edges of the HH~1 bow shock and of the brighter condensations of HH~2. These ridges have H$α$/H$β=4\to 6$, which is consistent with collisional excitation from the $n=1$ to the $n=3$ and 4 levels of hydrogen in a gas of temperatures $T=1.5\to 10\times 10^4$~K. This is therefore the first direct proof that the collisional excitation/ionization region of hydrogen right behind Herbig-Haro shock fronts is detected.

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arXiv2014

Interaction between the IGM and a dwarf galaxy

Dwarf Galaxies are the most common objects in the Universe and are believed to contain large amounts of dark matter. There are mainly three morphologic types of dwarf galaxies: dwarf ellipticals, dwarf spheroidals and dwarf irregulars. Dwarf irregular galaxies are particularly interesting in dwarf galaxy evolution, since dwarf spheroidal predecessors could have been very similar to them. Therefore, a mechanism linked to gas-loss in dwarf irregulars should be observed, i.e. ram pressure stripping. In this paper, we study the interaction between the ISM of a dwarf galaxy, and a flowing IGM. We derive the weak-shock, plasmon solution corresponding to the balance between the post-bow shock pressure and the pressure of the stratified ISM (which we assume follows the fixed stratification of a gravitationally dominant dark matter halo). We compare our model with previously published numerical simulations and with the observed shape of the HI cloud around the Ho II and Pegasus dwarf irregular galaxies. We show that such a comparison provides a straightforward way for estimating the Mach number of the impinging flow.

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arXiv2014

A two-mode planetary nebula luminosity function

We propose a new Planetary Nebula Luminosity Function (PNLF) that includes two populations in the distribution. Our PNLF is a direct extension of the canonical function proposed by Jacoby et al. (1987), in order to avoid problems related with the histogram construction, it is cast in terms of cumulative functions. We are interested in recovering the shape of the faint part of the PNLF in a consistent manner, for galaxies with and without a dip in their PN luminosity functions. The parameters for the two mode PNLF are obtained with a genetic algorithm, which obtains a best fit to the PNLF varying all of the parameters simultaneously in a broad parameter space. We explore a sample of 9 galaxies with various Hubble types and construct their PNLF. All of the irregular galaxies, except one, are found to be consistent with a two-mode population, while the situation is less clear for ellipticals and spirals.For the case of NGC\, 6822, we show that the two-mode PNLF is consistent with previous studies of the star formation history within that galaxy. Our results support two episodes of star formation, in which the latter is significantly stronger.

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arXiv2014

An asymmetric jet launching model for the protoplanetary nebula CRL 618

We propose an asymmetrical jet ejection mechanism in order to model the mirror symmetry observed in the lobe distribution of some protoplanetary nebulae (pPNe), such as the pPN CRL 618. 3D hydrodynamical simulations of a precessing jet launched from an orbiting source were carried out including an alternation in the ejections of the two outflow lobes, depending on which side of the precessing accretion disk is hit by the accretion column from a Roche lobe-filling binary companion. Both synthetic optical emission maps and position-velocity (PV) diagrams were obtained from the numerical results with the purpose of carrying out a direct comparison with observations. Depending on the observer's point of view, multipolar morphologies are obtained which exhibit a mirror symmetry at large distances from the central source. The obtained lobe sizes and their spatial distribution are in good agreement with the observed morphology of the pPN CRL 618. We also obtain that the kinematic ages of the fingers are similar to those obtained in the observations.

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arXiv2014

Proper Motions of Young Stellar Outflows in the Mid-Infrared with Spitzer. II. HH 377/Cep E

We have used multiple mid-infrared observations at 4.5 micron obtained with the Infrared Array Camera, of the compact (~1.4 arcmin) young stellar bipolar outflow Cep E to measure the proper motion of its brightest condensations. The images span a period of ~6 yr and have been reprocessed to achieve a higher angular resolution (~0.8 arcsec) than their normal beam (2 arcsec). We found that for a distance of 730 pc, the tangential velocities of the North and South outflow lobes are 62+/-29 and 94+/-6 km/s respectively, and moving away from the central source roughly along the major axis of the flow. A simple 3D hydrodynamical simulation of the H2 gas in a precessing outflow supports this idea. Observations and model confirm that the molecular Hydrogen gas, traced by the pure rotational transitions, moves at highly supersonic velocities without being dissociated. This suggests either a very efficient mechanism to reform H2 molecules along these shocks or the presence of some other mechanism (e.g. strong magnetic field) that shields the H2 gas.

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arXiv2014

Age, size, and position of H ii regions in the Galaxy. Expansion of ionized gas in turbulent molecular clouds

This work aims at improving the current understanding of the interaction between H ii regions and turbulent molecular clouds. We propose a new method to determine the age of a large sample of OB associations by investigating the development of their associated H ii regions in the surrounding turbulent medium. Using analytical solutions, one-dimensional (1D), and three-dimensional (3D) simulations, we constrained the expansion of the ionized bubble depending on the turbulent level of the parent molecular cloud. A grid of 1D simulations was then computed in order to build isochrone curves for H ii regions in a pressure-size diagram. This grid of models allowed to date large sample of OB associations and was used on the H ii Region Discovery Survey (HRDS). Analytical solutions and numerical simulations showed that the expansion of H ii regions is slowed down by the turbulence up to the point where the pressure of the ionized gas is in a quasi-equilibrium with the turbulent ram pressure. Based on this result, we built a grid of 1D models of the expansion of H ii regions in a profile based on Larson laws. The 3D turbulence is taken into account by an effective 1D temperature profile. The ages estimated by the isochrones of this grid agree well with literature values of well-known regions such as Rosette, RCW 36, RCW 79, and M16. We thus propose that this method can be used to give ages of young OB associations through the Galaxy such as the HRDS survey and also in nearby extra-galactic sources.

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arXiv2014

A Stellar Wind Origin for the G2 Cloud: Three-Dimensional Numerical Simulations

We present 3D, adaptive mesh refinement simulations of G2, a cloud of gas moving in a highly eccentric orbit towards the galactic center. We assume that G2 originates from a stellar wind interacting with the environment of the Sgr A* black hole. The stellar wind forms a cometary bubble which becomes increasingly elongated as the star approaches periastron. A few months after periastron passage, streams of material begin to accrete on the central black hole with accretion rates $\dot{M} \sim 10^{-8}$ M$_\odot$ yr$^{-1}$. Predicted Br$γ$ emission maps and position-velocity diagrams show an elongated emission resembling recent observations of G2. A large increase in luminosity is predicted by the emission coming from the shocked wind region during periastron passage. The observations, showing a constant Br$γ$ luminosity, remain puzzling, and are explained here assuming that the emission is dominated by the free-wind region. The observed Br$γ$ luminosity ($\sim 8 \times 10^{30}$ erg s$^{-1}$) is reproduced by a model with a $v_w=50$ km s$^{-1}$ wind velocity and a $10^{-7}$ M$_\odot$ yr$^{-1}$ mass loss rate if the emission comes from the shocked wind. A faster and less dense wind reproduces the Br$γ$ luminosity if the emission comes from the inner, free wind region. The extended cometary wind bubble, largely destroyed by the tidal interaction with the black hole, reforms a few years after periastron passage. As a result, the Br$γ$ emission is more compact after periastron passage.

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arXiv2014

Modelling the 3D morphology and proper motions of the planetary nebula NGC 6302

We present 3D hydrodynamical simulations of an isotropic fast wind interacting with a previously ejected toroidally-shaped slow wind in order to model both the observed morphology and the kinematics of the planetary nebula (PN) NGC 6302. This source, also known as the Butterfly nebula, presents one of the most complex morphologies ever observed in PNe. From our numerical simulations, we have obtained an intensity map for the H$α$ emission to make a comparison with the Hubble Space Telescope (HST) observations of this object. We have also carried out a proper motion (PM) study from our numerical results, in order to compare with previous observational studies. We have found that the two interacting stellar wind model reproduces well the morphology of NGC 6302, and while the PM in the models are similar to the observations, our results suggest that an acceleration mechanism is needed to explain the Hubble-type expansion found in HST observations.

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arXiv2013

The photoevaporation of a neutral structure by an EUV+FUV radiation field

The EUV photoionizing radiation and FUV dissociating radiation from newly born stars photoevaporate their parental neutral cloud, leading to the formation of dense clumps that could eventually form additional stars. We study the effects of including a photodissociating FUV flux in models of the fragmentation of a photoevaporating, self-gravitating molecular cloud. We compute 3D simulations of the interaction of an inhomogeneous, neutral, self-gravitating cloud with external EUV and FUV radiation fields, and calculate the number of collapsing clumps and their mass. We find that the presence of an outer photodissociation region has an important effect on the formation of dense structures due to the expansion of an HII region. In particular, including a FUV field leads to the earlier formation of a larger number of dense clumps, which might lead to the formation of more stars.

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arXiv2012

Gravitational drag on a point mass in hypersonic motion within a Gaussian disk

We develop an analytical model for the accretion and gravitational drag on a point mass that moves hypersonically in the midplane of a gaseous disk with a Gaussian vertical density stratification. Such a model is of interest for studying the interaction between a planet and a protoplanetary disk, as well as the dynamical decay of massive black holes in galactic nuclei. The model considers that the flow is ballistic, and gives fully analytical expressions for both the accretion rate onto the point mass, and the gravitational drag it suffers. The expressions are further simplified by taking the limits of a thick, and of a thin disk. The results for the thick disk reduce correctly to those for a uniform density environment (Cantó et al. 2011). We find that for a thin disk (small vertical scaleheight compared to the gravitational radius) the accretion rate is proportional to the mass of the moving object and to the surface density of the disk, while the drag force is independent of the velocity of the object. The gravitational deceleration of the hypersonic perturber in a thin disk was found to be independent of its parameters (i.e. mass or velocity) and depends only on the surface mass density of the disk. The predictions of the model are compared to the results of three-dimensional hydrodynamical simulations, with a reasonable agreement.

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arXiv2012

Proper Motions of Young Stellar Outflows in the Mid-Infrared with Spitzer (IRAC). I. The NGC 1333 region

We use two 4.5micron Spitzer (IRAC) maps of the NGC 1333 region taken over approx. 7 yr interval to determine proper motions of its associated outflows. This is a first, successful attempt at obtaining proper motions of stellars outflow from Spitzer observations. For the outflow formed by the Herbig-Haro objects HH7, 8 and 10, we find proper motions of approx. 9-13 km/s, which are consistent with previously determined optical proper motions of these objects. We determine proper motions for a total of 8 outflows, ranging from approx. 10 to 100 km/s. The derived proper motions show that out of these 8 outflows, 3 have tangential velocities less or equal to 20 km/s. This result shows that a large fraction of the observed outflows have low intrinsic velocities, and that the low proper motions are not merely a projection effect.

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arXiv2012

Spitzer observations of the HH 1/2 system. The discovery of the counterjet

We present unpublished Spitzer IRAC observations of the HH 1/2 young stellar outflow processed with a high angular resolution deconvolution algorithm that produces sub-arcsecond (approx. 0.6" - 0.8") images. In the resulting mid-infrared images the optically invisible counterjet is detected for the first time. The counterjet is approximately half as bright as the jet at 4.5 micron (the IRAC band that best traces young stellar outflows) and has a length of approx. 10". The NW optical jet itself can be followed back in the mid-IR to the position of the exciting VLA 1 source. An analysis of the IRAC colors indicates that the jet/counterjet emission is dominated by collisionally excited H2 pure rotational lines arising from a medium with a neutral Hydrogen gas density of 1000-2000 per cubic cm and a temperature of 1500 K. The observed jet/counterjet brightness asymmetry is consistent with an intrinsically symmetric outflow with extinction from a dense, circumstellar structure of 6" size (along the outflow axis), and with a mean visual extinction of Av=11 mag.

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