arXiv subjects

S. Curiel

Publications and source records attributed to S. Curiel.

34 records · Page 2Linked to original sources

arXiv2014

Rapidly increasing collimation and magnetic field changes of a H2O maser outflow

W75N(B) is a massive star-forming region that contains three radio continuum sources (VLA 1, VLA 2, and VLA 3), which are thought to be three massive young stellar objects at three different evolutionary stages. VLA 1 is the most evolved and VLA 2 the least evolved source. The 22 GHz H2O masers associated with VLA 1 and VLA 2 have been mapped at several epochs over eight years. While the H2O masers in VLA 1 show a persistent linear distribution along a radio jet, those in VLA 2 are distributed around an expanding shell. Furthermore, H2O maser polarimetric measurements revealed magnetic fields aligned with the two structures. Using new polarimetric observations of H2O masers, we aim to confirm the elliptical expansion of the shell-like structure around VLA 2 and, at the same time, to determine if the magnetic fields around the two sources have changed. The NRAO Very Long Baseline Array was used to measure the linear polarization and the Zeeman-splitting of the 22 GHz H2O masers towards the massive star-forming region W75N(B). The H2O maser distribution around VLA 1 is unchanged from that previously observed. We made an elliptical fit of the H2O masers around VLA 2. We find that the shell-like structure is still expanding along the direction parallel to the thermal radio jet of VLA 1. While the magnetic field around VLA 1 has not changed in the past 7 years, the magnetic field around VLA 2 has changed its orientation according to the new direction of the major-axis of the shell-like structure and it is now aligned with the magnetic field in VLA 1.

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arXiv2014

A very young, compact bipolar H$_2$O maser outflow in the intermediate-mass star-forming LkH$α$ 234 region

We report multi-epoch VLBI H$_2$O maser observations towards the compact cluster of YSOs close to the Herbig Be star LkH$α$ 234. This cluster includes LkH$α$ 234 and at least nine more YSOs that are formed within projected distances of $\sim$10 arcsec ($\sim$9,000 au). We detect H$_2$O maser emission towards four of these YSOs. In particular, our VLBI observations (including proper motion measurements) reveal a remarkable very compact ($\sim$0.2 arcsec = $\sim$180 au), bipolar H$_2$O maser outflow emerging from the embedded YSO VLA 2. We estimate a kinematic age of $\sim$40 yr for this bipolar outflow, with expanding velocities of $\sim$20 km s$^{-1}$ and momentum rate $\dot M_w V_w$ $\simeq$ $10^{-4}-10^{-3}$ M$_{\odot}$ yr$^{-1}$ km s$^{-1}$$\times (Ω$/$4π)$, powered by a YSO of a few solar masses. We propose that the outflow is produced by recurrent episodic jet ejections associated with the formation of this YSO. Short-lived episodic ejection events have previously been found towards high-mass YSOs. We show now that this behaviour is also present in intermediate-mass YSOs. These short-lived episodic ejections are probably related to episodic increases in the accretion rate, as observed in low-mass YSOs. We predict the presence of an accretion disk associated with VLA 2. If detected, this would represent one of the few known examples of intermediate-mass stars with a disk-YSO-jet system at scales of a few hundred au.

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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.

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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).

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arXiv2012

Water maser kinematics in massive star-forming regions: Cepheus A and W75N

VLBI multi-epoch water maser observations are a powerful tool to study the dense, warm shocked gas very close to massive protostars. The very high-angular resolution of these observations allow us to measure the proper motions of the masers in a few weeks, and together with the radial velocity, to determine their full kinematics. In this paper we present a summary of the main observational results obtained toward the massive star-forming regions of Cepheus A and W75N, among them: (i) the identification of different centers of high-mass star formation activity at scales of 100 AU; (ii) the discovery of new phenomena associated with the early stages of high-mass protostellar evolution (e.g., isotropic gas ejections); and (iii) the identification of the simultaneous presence of a wide-angle outflow and a highly collimated jet in the massive object Cep A HW2, similar to what is observed in some low-mass protostars. Some of the implications of these results in the study of high-mass star formation are discussed.

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arXiv2011

The structure of the magnetic field in the massive star-forming region W75N

A debated topic in star formation theory is the role of magnetic fields during the protostellar phase of high-mass stars. It is still unclear how magnetic fields influence the formation and dynamics of massive disks and outflows. Most current information on magnetic fields close to high-mass protostars comes from polarized maser emissions, which allows us to investigate the magnetic field on small scales by using very long-baseline interferometry. The massive star-forming region W75N contains three radio continuum sources (VLA1, VLA2, and VLA3), at three different evolutionary stages, and associated masers, while a large-scale molecular bipolar outflow is also present. Very recently, polarization observations of the 6.7 GHz methanol masers at milliarsecond resolution have been able to probe the strength and structure of the magnetic field over more than 2000 AU around VLA1. The magnetic field is parallel to the outflow, suggesting that VLA1 is its powering source. The observations of water masers at 22 GHz can give more information about the gas dynamics and the magnetic fields around VLA1 and VLA2. The NRAO Very Long Baseline Array was used to measure the linear polarization and the Zeeman-splitting of the 22 GHz water masers in the star-forming region W75N. We detected 124 water masers, 36 around VLA1 and 88 around VLA2 of W75N, which indicate two different physical environments around the two sources, where VLA1 is in a more evolved state. The linear polarization of the masers confirms the tightly ordered magnetic field around VLA1, which is aligned with the large-scale molecular outflow, and also reveals an ordered magnetic field around VLA2, which is not parallel to the outflow. [abridged]

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arXiv2010

Millimeter and submillimeter high angular resolution interferometric observations: dust in the heart of IRAS 18162-2048

The GGD27 complex includes the HH 80-81-80N system, which is one of the most powerful molecular outflows associated with a high mass star-forming region observed up to now. This outflow is powered by the star associated with the source IRAS 18162-2048. Here we report the detection of continuum emission at sub-arcsec/arcsec resolution with the Submillimeter Array at 1.36mm and 456microns, respectively. We detected dust emission arising from two compact cores, MM1 and MM2, separated by about 7" (~12000AU in projected distance). MM1 spatially coincides with the powerful thermal radio continuum jet that powers the very extended molecular outflow, while MM2 is associated with the protostar that drives the compact molecular outflow recently found in this region. High angular resolution obervations at 1.36mm show that MM1 is unresolved and that MM2 splits into two subcomponents separated by ~1". The mass of MM1 is about 4Msun and it has a size of <300AU. This is consistent with MM1 being associated with a massive and dense (n(H2)>10^9cm-3) circumstellar dusty disk surrounding a high-mass protostar, which has not developed yet a compact HII region. On the other hand, the masses of the two separate components of MM2 are about 2Msun each. One of these components is a compact core with an intermediate-mass young protostar inside and the other component is probably a pre-stellar core. MM1 is the brigthest source at 1.36mm, while MM2 dominates the emission at 456microns. These are the only (sub)millimeter sources detected in the SMA observations. Hence, it seems that both sources may contribute significantly to the bolometric luminosity of the region. Finally, we argue that the characteristics of these two sources indicate that MM2 is probably in an earlier evolutionary stage than MM1.

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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.

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arXiv2010

Statistical Analysis of Water Masers in Star-Forming Regions: Cepheus A and W75 N

We have done a statistical analysis of Very Long Baseline Array (VLBA) data of water masers in the star-forming regions (SFRs) Cepheus A and W75 N, using correlation functions to study the spatial clustering and Doppler-velocity distribution of these masers. Two-point spatial correlation functions show a characteristic scale size for clusters of water maser spots < or ~1 AU, similar to the values found in other SFRs. This suggests that the scale for water maser excitation tends to be < or ~1 AU. Velocity correlation functions show power-law dependences with indices that can be explained by regular velocity fields, such as expansion and/or rotation. These velocity fields are similar to those indicated by the water maser proper-motion measurements; therefore, the velocity correlation functions appear to reveal the organized motion of water maser spots on scales larger than 1 AU.

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arXiv2009

A simple algorithm for optimization and model fitting: AGA (asexual genetic algorithm)

Context. Mathematical optimization can be used as a computational tool to obtain the optimal solution to a given problem in a systematic and efficient way. For example, in twice-differentiable functions and problems with no constraints, the optimization consists of finding the points where the gradient of the objective function is zero and using the Hessian matrix to classify the type of each point. Sometimes, however it is impossible to compute these derivatives and other type of techniques must be employed such as the steepest descent/ascent method and more sophisticated methods such as those based on the evolutionary algorithms. Aims. We present a simple algorithm based on the idea of genetic algorithms (GA) for optimization. We refer to this algorithm as AGA (Asexual Genetic Algorithm) and apply it to two kinds of problems: the maximization of a function where classical methods fail and model fitting in astronomy. For the latter case, we minimize the chi-square function to estimate the parameters in two examples: the orbits of exoplanets by taking a set of radial velocity data, and the spectral energy distribution (SED) observed towards a YSO (Young Stellar Object). Methods. The algorithm AGA may also be called genetic, although it differs from standard genetic algorithms in two main aspects: a) the initial population is not encoded, and b) the new generations are constructed by asexual reproduction. Results. Applying our algorithm in optimizing some complicated functions, we find the global maxima within a few iterations. For model fitting to the orbits of exoplanets and the SED of a YSO, we estimate the parameters and their associated errors.

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arXiv2007

Multiple Sources toward the High-mass Young Star S140 IRS1

S140 IRS1 is a remarkable source where the radio source at the center of the main bipolar molecular outflow in the region is elongated perpendicular to the axis of the outflow, an orientation opposite to that expected if the radio source is a thermal jet exciting the outflow. We present results of 1.3 cm continuum and H2O maser emission observations made with the VLA in its A configuration toward this region. In addition, we also present results of continuum observations at 7 mm and re-analyse observations at 2, 3.5 and 6 cm (previously published). IRS 1A is detected at all wavelengths, showing an elongated structure. Three water maser spots are detected along the major axis of the radio source IRS 1A. We have also detected a new continuum source at 3.5 cm (IRS 1C) located ~0.6'' northeast of IRS 1A. The presence of these two YSOs (IRS 1A and 1C) could explain the existence of the two bipolar molecular outflows observed in the region. In addition, we have also detected three continuum clumps (IRS 1B, 1D and 1E) located along the major axis of IRS 1A. We discuss two possible models to explain the nature of IRS 1A: a thermal jet and an equatorial wind.

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arXiv2007

The Circumstellar Structure and Excitation Effects around the Massive Protostar Cepheus A HW 2

We report SMA 335 GHz continuum observations with angular resolution of ~0.''3, together with VLA ammonia observations with ~1'' resolution toward Cep A HW 2. We find that the flattened disk structure of the dust emission observed by Patel et al. is preserved at the 0.''3 scale, showing an elongated structure of ~$0.''6 size (450 AU) peaking on HW 2. In addition, two ammonia cores are observed, one associated with a hot-core previously reported, and an elongated core with a double peak separated by ~1.''3 and with signs of heating at the inner edges of the gas facing HW 2. The double-peaked ammonia structure, as well as the double-peaked CH3CN structure reported previously (and proposed to be two independent hot-cores), surround both the dust emission as well as the double-peaked SO2 disk structure found by Jimenez-Serra et al. All these results argue against the interpretation of the elongated dust-gas structure as due to a chance-superposition of different cores; instead, they imply that it is physically related to the central massive object within a disk-protostar-jet system.

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arXiv2004

On the Evolutionary State of the Components of the YLW 15 Binary System

We report centimeter continuum observations with the VLA and the VLBA as well as mid-infrared observations with COMICS/SUBARU toward the components of the YLW 15 very young binary system, VLA 1 and VLA 2. The centimeter emission of the two components traces partially thick free-free emission, likely due to collimated, ionized winds. VLA 1 is an embedded protostar, undetected in the near-IR, and possibly in the Class 0 to Class I transition and powering a Herbig-Haro outflow. Its mid-IR emission appears slightly resolved with a diameter of ~16 AU, possibly tracing circumstellar material from both the envelope and the disk. VLA 2 is a typical Class I object, unresolved in the mid-IR, and is the responsible of the strong X-ray emission associated with YLW 15. The expected centimeter ''peri-stellar'' emission associated with the X-ray emission is not detected with the VLBA at 6 cm likely due to the high optical depth of the free-free emission. Strikingly, the near to mid-IR properties of YLW 15 suggest that VLA 1 is a more embedded YSO, or alternatively, less luminous than VLA 2, whereas orbital proper motions of this binary system indicate that VLA 1 is more massive than VLA 2. This result is apparently against the expected evolutionary scenario, where one expects that the more massive YSO in a binary system is the more evolved and luminous YSO. Finally, the nearby source YLW 16A is detected with the VLA, its position coincides with reported near-IR and X-ray sources.

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arXiv2002

Radio Continuum Observations towards Optical and Molecular Outflows

We present multi-frequency VLA continuum observations towards 8 star forming regions with molecular and optical outflows: L1489, HH 68-69, HH 94-95, NGC 2264D, L1681B, L778, MWC 1080 and V645 Cyg. We detect three thermal radio jets, L1489, YLW 16A in L1681B and NGC 2264D VLA 7, associated with molecular and/or HH outflows. The L1489 and NGC 2264D VLA 7 thermal radio jets appear elongated in the direction of the larger scale outflow. We report the first tentative detection of a non-thermal radio jet, L778 VLA 5, associated with a low mass Class I protostar and powering a molecular outflow. For HH68-69, HH 94-95 and the molecular outflow in NGC 2264D we could not identify a candidate of the exciting source of these outflows. The radio emission associated with V645 Cyg is quite extended, ~0.1 pc, and time variable. We detect three radio sources in the MWC 1080 that could be associated with YSOs.

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arXiv2001

Kinematics of the HH 111 jet from STIS spectra

We present a long-slit spectrum of the Herbig-Haro object HH 111, obtained with SITS on the Hubble Space Telescope. This spectrum has a spectral resolution of approx. 50 km/s, so it gives a good picture of the kinematical properties of the observed object at very high angular resolution. We find that some of the knots along the jet are associated with sudden drops in the radial velocity (modulus), confirming that the emission from the knots is formed in shocks. We interprt the observed position-velocity diagrams in terms of a model of a jet from a variable source, and we attempt to carry out a reconstruction of the ejection variability necessary for reproducing the observed kinematical structure.

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arXiv2000

Optical and Near Infrared Study of the Cepheus E outflow, a very low excitation object

We present images and spectra of the Cepheus E (Cep E) region at both optical and infrared wavelengths. Only the brightest region of the southern lobe of the Cep E outflow reveals optical emission, suggesting that the extinction close to the outflow source plays an important rôle in the observed difference between the optical and IR morphologies. Cep E is a unique object since it provides a link between the spectroscopic properties of the optical Herbig-Haro (HH) objects and those of deeply embedded outflows.

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