arXiv subjects

Hector G. Arce

Publications and source records attributed to Hector G. Arce.

48 records · Page 3Linked to original sources

arXiv2010

The COMPLETE Survey of Outflows in Perseus

We present a study on the impact of molecular outflows in the Perseus molecular cloud complex using the COMPLETE survey large-scale 12CO(1-0) and 13CO(1-0) maps. We used three-dimensional isosurface models generated in RA-DEC-Velocity space to visualize the maps. This rendering of the molecular line data allowed for a rapid and efficient way to search for molecular outflows over a large (~ 16 sq. deg.) area. Our outflow-searching technique detected previously known molecular outflows as well as new candidate outflows. Most of these new outflow-related high-velocity features lie in regions that have been poorly studied before. These new outflow candidates more than double the amount of outflow mass, momentum, and kinetic energy in the Perseus cloud complex. Our results indicate that outflows have significant impact on the environment immediately surrounding localized regions of active star formation, but lack the energy needed to feed the observed turbulence in the entire Perseus complex. This implies that other energy sources, in addition to protostellar outflows, are responsible for turbulence on a global cloud scale in Perseus. We studied the impact of outflows in six regions with active star formation within Perseus of sizes in the range of 1 to 4 pc. We find that outflows have enough power to maintain the turbulence in these regions and enough momentum to disperse and unbind some mass from them. We found no correlation between outflow strength and star formation efficiency for the six different regions we studied, contrary to results of recent numerical simulations. The low fraction of gas that potentially could be ejected due to outflows suggests that additional mechanisms other than cloud dispersal by outflows are needed to explain low star formation efficiencies in clusters.

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arXiv2010

L1448 IRS2E: A candidate first hydrostatic core

Intermediate between the prestellar and Class 0 protostellar phases, the first core is a quasi-equilibrium hydrostatic object with a short lifetime and an extremely low luminosity. Recent MHD simulations suggest that the first core can even drive a molecular outflow before the formation of the second core (i.e., protostar). Using the Submillimeter Array and the Spitzer Space Telescope, we present high angular resolution observations towards the embedded dense core IRS2E in L1448. We find that source L1448 IRS2E is not visible in the sensitive Spitzer infrared images (at wavelengths from 3.6 to 70 um), and has weak (sub-)millimeter dust continuum emission. Consequently, this source has an extremely low bolometric luminosity (< 0.1 L_sun). Infrared and (sub-)millimeter observations clearly show an outflow emanating from this source; L1448 IRS2E represents thus far the lowest luminosity source known to be driving a molecular outflow. Comparisons with prestellar cores and Class 0 protostars suggest that L1448 IRS2E is more evolved than prestellar cores but less evolved than Class 0 protostars, i.e., at a stage intermediate between prestellar cores and Class 0 protostars. All these results are consistent with the theoretical predictions of the radiative/magneto hydrodynamical simulations, making L1448 IRS2E the most promising candidate of the first hydrostatic core revealed so far.

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arXiv2006

The Evolution of Outflow-Envelope Interactions in Low-Mass Protostars

We present multi-line and continuum observations of the circumstellar environment within 10^4 AU of a sample of protostars to investigate how the effects of outflows on their immediate environment changes over time. 12CO(1-0) emission probes the high-velocity molecular outflows near the protostars and demonstrate that the outflow opening angle widens as the nascent star evolves. Maps of the 13CO(1-0) and HCO+(1-0) outflow emission show that protostellar winds erode the circumstellar envelope through the entrainment of the outer envelope gas. The spatial and velocity distribution of the dense circumstellar envelope, as well as its mass, is traced by the C18O(1-0) emission and also displays evolutionary changes. We show that outflows are largely responsible for these changes, and propose an empirical model for the evolution of outflow-envelope interactions. In addition, some of the outflows in our sample appear to affect the chemical composition of the surrounding environment, enhancing the HCO+ abundance. Overall, our results confirm that outflows play a major role in the star formation process through their strong physical and chemical impacts on the environments of the young protostars.

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arXiv2006

Molecular Outflows in Low- and High-Mass Star Forming Regions

We review the known properties of molecular outflows from low- and high-mass young stars. General trends among outflows are identified, and the most recent studies on the morphology, kinematics, energetics, and evolution of molecular outflows are discussed, focusing on results from high-resolution millimeter observations. We review the existing four broad classes of outflow models and compare numerical simulations with the observational data. A single class of models cannot explain the range of morphological and kinematic properties that are observed, and we propose a possible solution. The impact of outflows on their cloud is examined, and we review how outflows can disrupt their surrounding environment, through the clearing of gas and the injection of momentum and energy onto the gas at distances from their powering sources from about 0.01 to a few pc. We also discuss the effects of shock-induced chemical processes on the ambient medium, and how these processes may act as a chemical clock to date outflows. Lastly, future outflow research with existing and planned millimeter and submillimeter instruments is presented.

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arXiv2004

PV Ceph: Young Star Caught Speeding?

Three independent lines of evidence imply that the young star PV Ceph is moving at roughly 20 km/s through the interstellar medium. The first, and strongest, suggestion of motion comes from the geometry of the HH knots in the "giant" Herbig-Haro (HH) flow associated with PV Ceph. Bisectors of lines drawn between pairs of knots at nearly equal distances from PV Ceph imply an E-W motion of the source, and a plasmon model fit to the knot positions gives a good fit of 22 km/s motion for the star. The second bit of damning evidence comes from a redshifted "trail" of molecular gas, pointing in the same E-W direction implied by the HH knot geometry. The third exhibit we offer in accusing PV Ceph of speeding involves the tilt apparent in the high-velocity molecular jet now emanating from the star. This tilt is best explained if the true, current, jet direction is N-S, as it is in HST images, and the star is moving at roughly 20 km/s. Tracing the motion of PV Ceph backward in time, to the nearest cluster from which it might have been ejected, we find that it is very likely to have been thrown out of the massive star-forming cluster NGC 7023 (more than 10 pc away). We propose that PV Ceph was ejected, at a speed large enough to escape NGC 7023, at least 100,000 years ago, but that it did not enter the molecular cloud in which it now finds itself until more like 35,000 years ago. Our calculations show that the currently-observable molecular outflow associated with PV Ceph is about 10,000 years old, so that the flow has had plenty of time to form while in its current molecular cloud. But, the question of what PV Ceph was doing, and what gas/disk it took along with it in the time it was traveling through the low-density region between NGC 7023 and its current home is an open question.

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arXiv2002

Bow shocks, Wiggling Jets, and Wide-Angle Winds: A High Resolution Study of the Entrainment Mechanism of the PV Ceph Molecular (CO) Outflow

We present a new set of high-resolution molecular line maps of the gas immediately surrounding various Herbig-Haro (HH) knots of the giant HH flow HH 315, from the young star PV Cephei. The observations, aimed at studying the entrainment mechanism of the 2.6 pc-long HH 315 flow, include IRAM 30m maps of the 12CO(2-1), 12CO(1-0), and 13CO(1-0) lines, with beam sizes of 11'', 21'', and 22'', respectively. We compare the morphology and the kinematics of the outflow gas, as well as the temperature and momentum distribution of the molecular outflow with those predicted by different entrainment models. With our detailed study we are able to conclude that jet bow shock entrainment by an episodic stellar wind, with a time-varying axis, produces most of the high-velocity molecular outflow observed far from the source. In addition, near PV Cephei we find evidence for a poorly collimated, wide-angle, molecular outflow and a collimated wiggling jet-like molecular outflow. We propose that the poorly collimated component is entrained by a wide-angle wind, and the collimated component is entrained by a variable jet with internal working surfaces. If this picture is true, then a stellar wind model which allows for the coexistence of a wide-angle component and a collimated (jet-like) stellar wind component is needed to explain the observed properties of the PV Ceph outflow. The wiggling axis of the redshifted molecular outflow lobe indicates that the outflow ejection axis is changing over time. We find that the time-scale of the axis variation shown by the molecular outflow lobe is about a factor of 10 less than that shown by the large-scale optical HH knots.

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arXiv2002

The Great PV Ceph Outflow: A Case Study in Outflow-Cloud Interaction

We present a set of detailed molecular line maps of the region associated with the giant Herbig-Haro flow HH 315, from the young star PV Cephei, aimed at studying the outflow-cloud interaction. Our study clearly shows that the HH 315 flow is effecting the kinematics of its surrounding medium, and has been able to redistribute considerable amounts of the surrounding medium-density gas in its star-forming core as well at parsec-scale distances from the source. The giant molecular outflow HH 315 is a highly asymmetric bipolar flow with a projected linear extent of about 2 pc. Our results indicate that the two outflow lobes are each interacting with the ambient medium in different ways. The southern (redshifted) lobe interacts with a dense ambient medium, very close to the young stellar outflow source, and its kinetic energy is comparable to both the turbulent and gravitational binding energy of its host cloud. In addition, we find evidence that the southern lobe is responsible for the creation of a cavity in the 13CO emission. In contrast, the northern (mainly blueshifted) outflow lobe extends farther from PV Ceph and interacts with ambient gas much less dense than the southern lobe. There is very little 13CO emission north of the outflow source, and the only prominent 13CO emission is a shell-like structure coincident with the outer edge of the northern lobe, about 1.2 pc northwest of PV Ceph. It appears that the northern lobe of the HH 315 outflow has been able to ``push'' aside a substantial fraction of the gas in the area, piling it in a dense shell-like structure at its edges. In addition, we find that the northern outflow lobe is responsible for a velocity gradient in the ambient gas.

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arXiv2001

The Mass-Velocity and Position-Velocity Relations in Episodic Outflows

While observational evidence for the episodic nature of young stellar outflows continues to mount, existing numerical and theoretical models of molecular outflows assume they are formed by the interaction of a non-episodic wind from a young stellar object with an ambient cloud. In this Letter we estimate and discuss the effects of episodicity on the mass-velocity and position-velocity relations observed in molecular outflows. We explain how many recent observational results disagree with the predictions of non-episodic outflow models, and we offer simple explanations for the discrepancies. In particular, we discuss how an episodic stellar outflow can steepen the power-law slope of the mass-velocity relation in a molecular outflow. And, we illustrate how an episodic outflow can produce multiple "Hubble-wedges'' in the position-velocity distribution of a molecular outflow. With a little more information than we have now, it may be possible to use the "fossil record" embedded in a molecular outflow's mass-velocity and position-velocity relations to reconstruct the history of a young stellar object's mass ejection episodes.

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arXiv2001

The Episodic, Precessing Giant Molecular Outflow from IRAS 04239+2436 (HH 300)

We present the first set of detailed molecular line maps of the region associated with the giant Herbig-Haro flow HH 300, from the young star IRAS 04239+2436. Our results indicate that the red lobe of the HH 300 flow is depositing a fair amount of momentum and kinetic energy over a notable volume of its host dark cloud. This makes HH 300 a key player in the evolution and fate of its parent cloud. The redshifted molecular outflow lobe of HH 300 is 1.1 pc long and 0.3 pc wide, and has a very clumpy structure. The density, velocity, and momentum distributions in the outflow indicate that the observed clumps arise from the prompt entrainment of ambient gas. Bow shock-shaped structures are associated with the clumps, and we hypothesize that the shocks are produced by different mass ejection episodes. Lines drawn from IRAS 04239+2436 to each of these clumps have different orientations on the plane of the sky, and we conclude that HH 300 is a precessing and episodic outflow. The observations include a map of the red lobe in the 12CO(2-1) line, with a beam size of 27'', and more extended maps of the outflow region in the 12CO(1-0) and 13CO(1-0) lines, with 45'' and 47'' beam sizes, respectively. We obtain a steep power-law mass spectrum for HH 300, which we believe is best explained by the evolution of the outflow mass kinematics. In addition, our 13CO observations show that the HH 300 flow has been able to redistribute (in space and velocity) considerable amounts of its surrounding medium-density gas.

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arXiv1999

An Extinction Study of the Taurus Dark Cloud Complex

We present a study of the detailed distribution of extinction in a region of the Taurus dark cloud complex. Our study uses new BVR images of the region, spectral classification data for 95 stars, and IRAS Sky Survey Atlas (ISSA) 60 and 100 micron images. We study the extinction of the region in four different ways, and we present the first inter-comparison of all these methods, which are: 1) using the color excess of background stars for which spectral types are known; 2) using the ISSA 60 and 100 micron images; 3) using star counts; and 4) using an optical (V and R) version of the average color excess method used by Lada et al. (1994). We find that all four methods give generally similar results, with important exceptions. To study the structure in the dust distribution, we compare the ISSA extinction and the extinction measured for individual stars. From the comparison, we conclude that in the relatively low extinction regions studied, with 0.9 < A_V < 3.0 mag (away from filamentary dark clouds and IRAS cores), there are no fluctuations in the dust column density greater than 45% (at the 99.7% confidence level), on scales smaller than 0.2 pc. We also report the discovery of a previously unknown stellar cluster behind the Taurus dark cloud near R.A 4h19m00s, Dec. 27:30:00 (B1950)

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arXiv1999

Measuring Galactic Extinction: A Test

We test the recently published all-sky reddening map of Schlegel, Finkbeiner & Davis (1998 [SFD]) using the extinction study of a region in the Taurus dark cloud complex by Arce & Goodman (1999 [AG]). In their study, AG use four different techniques to measure the amount and structure of the extinction toward Taurus, and all four techniques agree very well. Thus we believe that the AG results are a truthful representation of the extinction in the region and can be used to test the reliability of the SFD reddening map. The results of our test show that the SFD all-sky reddening map, which is based on data from COBE/DIRBE and IRAS/ISSA, overestimates the reddening by a factor of 1.3 to 1.5 in regions of smooth extinction with A_V > 0.5 mag. In some regions of steep extinction gradients the SFD map underestimates the reddening value, probably due to its low spatial resolution. We expect that the astronomical community will be using the SFD reddening map extensively. We offer this Letter as a cautionary note about using the SFD map in regions of high extinction (A_V > 0.5 mag), as it might not be giving accurate reddening values there.

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arXiv1998

The Polarizing Power of the Interstellar Medium in Taurus

We present a study of the polarizing power of the dust in cold dense regions (dark clouds) compared to that of dust in the general interstellar medium (ISM). Our study uses new polarimetric, optical, and spectral classification data for 36 stars to carefully study the relation between polarization percentage (p) and extinction (A_V) in the Taurus dark cloud complex. We find two trends in our p-A_V study: (1) stars background to the warm ISM show an increase in p with A_V; and (2) the percentage of polarization of stars background to cold dark clouds does not increase with extinction. We detect a break in the p-A_V relation at an extinction 1.3 +/- 0.2 mag, which we expect corresponds to a set of conditions where the polarizing power of the dust associated with the Taurus dark clouds drops precipitously. This breakpoint places important restrictions on the use of polarimetry in studying interstellar magnetic fields.

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