INTERACTION MODEL FOR HH 505

Transcription

1 THE ASTRONOMICAL JOURNAL, 121:408È412, 2001 January ( The American Astronomical Society. All rights reserved. Printed in U.S.A. A JETÈSIDE WIND INTERACTION MODEL FOR THE CURVED JETS IN THE ORION NEBULA E. MASCIADRI AND A. C. RAGA Instituto de Astronom a, Universidad Nacional Auto noma de Me xico, Apdo. Postal , Me xico D.F., Mexico Received 2000 September 13; accepted 2000 October 2 ABSTRACT Bally & Reipurth have recently discovered a number of Herbig-Haro (HH) jets in the periphery of the Orion Nebula (M42). In many cases both these jets and their counterjets curve away from the central regions of M42. These curved outñows appear to be evidence of an interaction of the jets with an outward Ñow in the H II region. In this paper, we present three-dimensional gasdynamic models of HH jetèside wind interactions, showing that structures similar to those observed in the newly discovered jets are straightforwardly obtained. In particular, we concentrate on modeling the HH 505 jet and obtain one of the best agreements ever reported with gasdynamic models for emission-line maps of HH objects. Key words: H II regions È ISM: Herbig-Haro objects 1. INTRODUCTION Head-tail ÏÏ radio jets have been modeled as the interaction of a collimated bipolar ejection with a side-sweeping intergalactic medium (Begelman et al. 1979; Balsara & Norman 1992), and more complex interactions with a rotating disk ÏÏ environment have been studied by Lim & Ste en (2000). Similar models have been explored in the context of Herbig-Haro (HH) systems that show both the jet and the counterjet curving in the same direction. This kind of morphology suggests the presence of a relative motion of the outñow source with respect to the surrounding ISM. Both analytic models (Canto & Raga 1995) and threedimensional numerical simulations (Lim & Raga 1998) of HH jets interacting with a side wind have been computed. However, such models have not been pursued in detail, because there were few HH jets showing clear curvatures. A better example of a jetèside wind interaction was provided by HH 30 (Lo pez et al. 1995, 1996), which actually does not exhibit a very impressive curvature. The situation has changed rather dramatically with the paper of Bally & Reipurth (2001), who have discovered several HH jets in the Orion Nebula (M42), some of which correspond to jets and counterjets with clearly deðned curvatures away from the center of the H II region. Bally & Reipurth (2001) interpret these observations as direct evidence of the e ect of the expanding H II region sweeping by and interacting with the HH jets. These authors Ðnd a different situation in NGC 1333, where they have discovered jets with curvatures toward the center of the stellar cluster. They interpret this as evidence for motions of the HH jet sources away from the cluster. These impressive new observations have motivated us to compute new three-dimensional gasdynamic models of jetè side wind interactions to see if the structures observed in these objects do correspond to the predictions from such models. In particular, we concentrate on modeling HH 505. This system has a well-deðned, approximately northward directed jet and a less well deðned, oppositely directed counterjet. Both the jet and the counterjet curve to the west, approximately deðning the arc of a circle with a radius of curvature r B 50A. By trial and c error, we have found a set of parameters for a three-dimensional gasdynamic model that reproduces the observed curvature of the HH 505 jet, as described in 2. The results from this model are presented in 3. A comparison between the predicted and the observed Ha maps is made, and simulations carried out with other Ñow parameters are discussed, illustrating the properties expected for other jets with parameters similar to those of HH 505. Finally, the successes and failures of the model are discussed in THE PARAMETERS FOR A JETÈSIDE WIND INTERACTION MODEL FOR HH 505 Bally & Reipurth (2001) have obtained Ha and red [S II] images of HH 505, as well as long-slit spectroscopic data. The HH 505 jet (which is directed approximately northward) shows one knot (N1) at D6A.5 from the source (other faint aligned knots being marginally detected in this region) and then continues as a more di use curved structure ending in a head ÏÏ (N3 in Bally & Reipurth 2001) at D40A from the source. Because there are yet no measured proper motions, we are forced to assume a reasonable ÏÏ value of v \ 100 km j s~1 for the jet velocity. On this velocity, we superpose a sinusoidal ejection velocity variability of period q \ 60 yr and half-amplitude *v \ 20 km s~1 in order to produce knots similar to the ones observed close to the source of the HH 505 jet. We assume that the jet has a top-hat initial cross section with a radius r \ 2.5 ] 1015 cm, corresponding to 0A.35 at the distance of j M42 (470 pc), which is comparable to the 0A.3 jet radius estimated by Bally & Reipurth (2001) for HH 505. For the initial density of the jet we choose n \ 103 cm~3, which is comparable to the B1300 cm~3 electron j density found for the base of the HH 505 jet by Bally & Reipurth (2001). We should note that because the HH 505 jet beam is externally photoionized, the values of the electron density and of the atom-plus-ion number density should be comparable (as opposed to normal ÏÏ HH jets, in which the ionization fraction is low). We then inject this jet into a plane-parallel wind of density n \ 180 cm~3 and velocity v \ 15 km s~1 moving perpendicularly w to the initial direction w of the jet. About these parameters, we can only say that they do not seem to be unreasonable for the photoionized gas in the periphery of M42. We have also explored models with other wind parameters, and we include an example of such a model in Figure 3 below (also see 3). 408

2 CURVED JETS IN THE ORION NEBULA 409 Finally, we assume that the whole computational domain is optically thin to the ionizing radiation from h Orionis (the validity of this approximation being evident from the analytic photoionized jet model of Raga et al. 2000b), which we model as a blackbody of T \ 3.5 ] 104 K, producing S \ 7.28 ] 1048 s~1 ionizing eff photons per unit time at a distance * D \ 4 ] 1018 cm from the jet. With these parameters, we use the simpliðed treatment of Canto et al. (1998) to compute the photoionization rate and the associated heating rate per neutral atom, and we incorporate these rates in our gasdynamic code (see below). We then set the initial temperature to a value of 104 K and the neutral hydrogen fraction to 10~3 for both the wind and the jet, and these parameters rapidly relax to photoionization equilibrium with the impinging ionizing photon Ñux. We integrate the three-dimensional gasdynamic equations (plus a rate equation for neutral hydrogen) with the YGUAZUŠ -AÏÏcode. This code is described in detail by Raga, Navarro-Gonza lez, & Villagra n-muniz (2000c), and the HH jet version ÏÏ of the code (which we use for the present calculation) is described by Raga et al. (2000a). This HH jet version of the code uses the computed neutral hydrogen fraction, the density, and the temperature of the Ñow to calculate a parameterized cooling function (described by Raga et al. 1999), which is then included as a sink term in the energy equation. We have now also added the photoionization rate and the associated heating rate using the approximate formalism described by Canto et al. (1998; see also above). With this code and with the parameters described above, we carry out a time integration that gives the evolution of the model jet. For this simulation, we use a four-level binary adaptive grid with a maximum resolution of 1.5 ] 1015 cm (in the three dimensions). 3. DISCUSSION In Figure 1, we show the density stratiðcation, the velocity Ðeld, and the conðguration of the adaptive grid (chosen by the adaptive grid algorithm of the YGUAZUŠ -A code; see Raga et al. 2000c) for a t \ 2250 yr time integration. The density stratiðcation shows the curved structure of the jet and the stagnation region of a sideways bow shock, which is formed by the lateral wind as it sweeps past the jet beam. The curved jet morphology is also evident in the velocity structure of the Ñow (see Fig. 1, middle). In Figure 2, we show a time series of the Ha maps predicted from the numerical model (computed assuming that the direction of the sideways wind and the initial direction of the jet both lie on the plane of the sky). Also shown in Figure 2 is the Ha image of HH 505 reported by Bally & Reipurth (2001). The agreement between the observed and the predicted Ha maps is very good. Even though the model parameters have been adjusted so as to reproduce the observed curvature of the HH 505 jet, it is clear that the observed and predicted images agree in surprising detail: 1. Close to the source (at an angular distance less than 15A), we have emission both from the sideways bow shock and from the knots along the jet beam. 2. At larger distances from the source, the emission from the sideways bow shock becomes dominant. 3. The head of the jet is asymmetric, with a faint extended wing in the downwind direction. 4. A break ÏÏ in the curved jet structure is present at a distance from the source of about two-thirds of the jet length. This last feature appears in the simulation because of the following e ect: The leading working surface has strong radiative shocks, which compress the preshock material into a dense clump.ïï The higher inertia of this clump (as compared with that of the lower density material along the jet beam) gives it a straighter trajectory in the presence of the sideways drag of the impinging wind. The leading working surface thus becomes decoupled from the main body of the jet and continues traveling along its own trajectory as a disconnected bullet.ïï Because of the lack of a resupply of momentum from the body of the jet, this isolated working surface slows down considerably as it incorporates low-momentum material from the surrounding environment (as can be appreciated in the velocity Ðeld shown in Fig. 1, in which one sees that the Ñow velocity of FIG. 1.ÈDensity stratiðcation on the plane including the direction of the side wind, which impinges from the left, and the ejection axis of the jet (left), vectors showing the velocity Ðeld on this plane (middle), and the structure of the adaptive grid chosen by the YGUAZUŠ -Acode (right) obtained from the model for HH 505 (see text) for a t \ 2250 yr time integration. The computational domain is of 2 ] 1017 cm (abscissa) times 2.5 ] 1017 cm (ordinate). The density stratiðcation is shown with linearly spaced isolines with a step of 2 ] 10~22 gcm~3. The longest arrows in the vector velocity map correspond to a velocity of 100 km s~1. Of the four levels of the adaptive grid, two are deðned over the whole computational domain, and the two higher levels of reðnement can be seen as regions of larger density of grid points.

3 410 MASCIADRI & RAGA Vol. 121 FIG. 2.ÈComparison between simulated and observed Ha intensity maps of HH 505. The top right panel shows the observed map of the HH 505 jet (with the source being the star seen at the lower left and with the counterjet falling outside the plot), which we have rotated 10 to the east so that the initial direction of the jet beam is approximately parallel to the ordinate. The remaining panels show the Ha maps obtained from a numerical simulation for di erent integration times (each panel being labeled with the time in years), assuming that both the impinging wind and the initial direction of the jet lie on the plane of the sky. The observed Ha map is depicted with a linear gray scale with arbitrary units (top right). All of the predicted maps are depicted with the same linear gray scale, which is given (in units of ergs cm~2 s~1 sr~1) in the bottom right plot. In the three-dimensional simulation, a jet is injected into the computational grid parallel to the ordinate and travels into a plane-parallel wind that moves along the abscissa. As the jet travels into the computational domain, it is deñected by the interaction with the laterally impinging wind. The physical parameters of the model are discussed in the text. the leading working surface is considerably lower than the jet velocity). An interesting problem that can be analyzed with numerical simulations is the sensitivity of jetèside wind interaction models to the physical parameters of the model. In Figure 3, we show the results obtained from simulations in which some of the parameters have been changed. In this Ðgure, we show models with initial jet density n \ 1000 j cm~3, average injection velocity v \ 100 km s~1 (i.e., j identical to the parameters of the HH 505 model described above), and other parameters as follows: 1. A velocity time variability with q \ 180 yr and *v \ 60 km s~1, and a side wind with n \ 180 cm~3 and v \ 15 km s~1 (Fig. 3, left); 2. A velocity time variability with q \ 60 yr and *v \ 60 km s~1, and a side wind with n \ 180 cm~3 and v \ 15 km s~1 (Fig. 3, middle);

4 No. 1, 2001 CURVED JETS IN THE ORION NEBULA 411 FIG. 3.ÈDensity on the midplane of the simulation (top) and Ha maps (bottom) obtained from simulations with di erent physical parameters. For each model, a di erent time slice was chosen, so that in all cases the jet is about to start leaving the computational domain. Starting from the left, we show models with: (1) a velocity time variability with q \ 180 yr and *v \ 60 km s~1, and a side wind with n \ 180 cm~3 and v \ 15 km s~1; (2) a velocity time variability with q \ 60 yr and *v \ 60 km s~1, and a side wind with n \ 180 cm~3 and v \ 15 km w s~1; (3) a velocity w time variability with q \ 60 yr and *v \ 20 km s~1, and a side wind with n \ 180 cm~3 and v \ 40 km w s~1. The linear gray w scales are shown on the right side of the plots (in units of g cm~3 for the density and in ergs cm~2 s~1 sr~1 w for the Ha maps). w 3. A velocity time variability with q \ 60 yr and *v \ 20 km s~1, and a side wind with n \ 180 cm~3 and v \ 40 km s~1 (Fig. 3, right). We have chosen appropriate time slices for each of the models so that in all cases the jet is about to start leaving the computational domain. Model 2 is identical to our model for HH 505, except that the amplitude of the velocity variability is 3 times larger. Interestingly, a comparison between the bottom center panel of Figure 3 and the Ha maps for the HH 505 model (see Fig. 2) shows that the knots along the jet do not have a drastically di erent Ha intensity. This lack of sensitivity to the amplitude of the velocity variability is di erent from the result that would be obtained from a normal ÏÏ HH jet model, in which the Ha emission of the knots is proportional to a power of D3È4 of the shock velocity (see Kofman & Raga 1992). In the present models, the dependence on the shock velocity is considerably lower, because most of the ionization is the direct result of photoionization (rather than of the shock heating). This relative lack of sensitivity of the emission to the shock velocities appears to be a special characteristic of photoionized jets. The predicted Ha intensity maps obtained from model 1 show knots with larger separations. In this simulation, the knots become highly asymmetric as they travel away from the source, and their emission becomes progressively merged with the emission from the sideways bow shock of the jetèside wind interaction (Fig. 3, bottom left). Finally, model 3 has a jet identical to that of our HH 505 model (see above and Fig. 2), but its side wind is 3 times faster. This change in the velocity of the impinging wind has a drastic e ect, producing a much shorter radius of curvature for the locus of the jet beam. Also, the higher Mach number of the lateral wind produces a considerably smaller stando distance between the sideways bow shock and the jet beam. 4. CONCLUSIONS We have shown that one can successfully model the morphology of the Ha intensity map of the HH 505 jet with a jetèside wind interaction gasdynamic model. Unfortunately, the model has many free parameters, because many of the physical parameters of the HH 505 system have still not been measured. There are yet no radial velocity or proper motion measurements, nor are there estimates of the density and velocity of the surrounding environment (though, of course, these parameters will be obtained with future observations of this object). The parameters that we have chosen for the jet are not unreasonable considering the known characteristics of other HH objects. Regarding the parameters for the side wind, it is unclear exactly what are the real values of its electron density and its velocity. We have

5 412 MASCIADRI & RAGA chosen n \ 180 cm~3 and v \ 15 km s~1, but other pairs of values w providing the same w postshock pressure (through the weak sideways bow shock) will produce similar curvatures for the path of the jet beam, so that in principle they would also be possible choices. Our model is also consistent with the observations in the following way: From Figure 2, it is clear that the model can reproduce the relative Ha intensities of the di erent regions of the HH 505 jet. We Ðnd that the absolute values of the intensities are also in the correct range. For example, the Ha emission from the head of the jet at time t \ 2250 yr (the time frame that better reproduces the observed structure of the HH 505 jet; see Fig. 2) has a value Ha \ 2.3 ] 10~15 ergs cm~2 arcsec~2. This value is approximately a factor of 7 lower than that (1.6 ] 10~14 ergs cm~2 arcsec~2) obtained by taking the average of the values reported for the N2 and N3 knots by Bally & Reipurth (2001). Such a value for the Ha intensity could be obtained from our model by scaling up the densities or the Ñow velocities by an appropriate amount. Finally, we note an interesting e ect. A stando distance between the sideways bow shock and the jet beam of B3È5A is observed in the HH 505 jet. This value is a factor of D5È10 larger than the radius of the jet beam. As is clear from Figure 2, our HH 505 model has an appreciably smaller stando distance, even though the Mach number of the lateral wind has a low (M \ 1.6) value. Models with higher wind Mach numbers have w even lower stando distances (see Fig. 3). From this, we would conclude that the Mach number of the lateral wind in HH 505 must have a value very close to unity. Alternatively, it is possible that the HH 505 jet could have a broader Ñow enveloping the observed jet beam. This could be, e.g., the result of the presence of a less well collimated stellar wind coexisting with the highly collimated jet. Future kinematic observations of this object might show whether or not such a broader wind is actually present. We are most grateful to John Bally and Bo Reipurth for kindly giving us their Ha image of HH 505. The work of A. C. R. was supported by CONACYT grants E and E and by a fellowship from the John Simon Guggenheim Foundation. The work of E. M. was supported by CONACYT grants J32412-E and E and DGAPA grant IN REFERENCES Bally, J., & Reipurth, B. 2001, ApJ, 546, 000 Lo pez, R., Riera, A., Raga, A. C., Anglada, G., Lo pez, A., Noriega-Crespo, Balsara, D. S., & Norman, M. L. 1992, ApJ, 393, 631 A., & Estalella, R. 1996, MNRAS, 282, 470 Begelman, M. C., Rees, M. J., & Blandford, R. D. 1979, Nature, 279, 770 Raga, A. C., Curiel, S., Rodr guez, L. F., & Canto, J. 2000a, A&A, in press Canto, J., & Raga, A. C. 1995, MNRAS, 277, 1120 Raga, A. C., et al. 2000b, MNRAS, 314, 681 Canto, J., Raga, A. C., Ste en, W., & Shapiro, P. 1998, ApJ, 502, 695 Raga, A. C., Mellema, G., Arthur, S. J., Binette, L., Ferruit, P., & Ste en, W. Kofman, L., & Raga, A. C. 1992, ApJ, 390, , Rev. Mexicana Astron. AstroÐs., 35, 123 Lim, A. J., & Raga, A. C. 1998, MNRAS, 298, 871 Raga, A. C., Navarro-Gonza lez, R., & Villagra n-muniz, M. 2000c, Rev. Lim, A. J., & Ste en, W. 2000, MNRAS, in press Mexicana Astron. AstroÐs., 36, 67 Lo pez, R., Raga, A. C., Riera, A., Anglada, G., & Estalella, R. 1995, MNRAS, 274, L19