Laboratory Analog of Heavy Jets Impacting a Denser Medium inHerbig–Haro (HH) Objects

G. Y. Liang1 , J. Y. Zhong2, H. G. Wei1, D. W. Yuan1, Z. Zhang3, C. Wang4, B. Han2, B. J. Zhu3, W. M. Jiang3, J. M. Peng1,2,T. Tao5, G. Y. Hu5, F. L. Wang1, X. Gao6, B. Q. Zhu7, J. Q. Zhu7, X. W. Ma8, Y. T. Li3,9, G. Zhao1, and J. Zhang7

1 Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012,People’s Republic of China; gyliang@bao.ac.cn, gzhao@bao.ac.cn

2 Department of Astronomy, Beijing Normal University, Beijing 100875, People’s Republic of China3 National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China

4 Shanghai Institute of Laser Plasma, Shanghai 201800, People’s Republic of China5 Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China

6 Beijing Computational Science Research Center, Beijing 100193, People’s Republic of China7 National Laboratory on High Power Laser and Physics, Shanghai 201800, People’s Republic of China

8 Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China9 School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China

Received 2018 July 31; revised 2018 September 13; accepted 2018 October 11; published 2018 November 20

Abstract

X-ray emissions in protostars play an important role in the chemistry of protostellar disks and in constraining thephysics of jet formation. We have experimentally investigated the mechanism of X-ray emission in protostellar jetsand modeled their interaction with the surrounding medium. The simulated supersonic jet is generated by intenselaser beams irradiating a K-shaped target and then impacts an obstacle. We have successfully observed X-rayemission from the obstacle surface, and we find that it comes from the outflow material and not completely fromthe ambient medium heated by shock.

Key words: ISM: jets and outflows – methods: laboratory: atomic – plasmas – X-rays: general

1. Introduction

Since the pioneering work of Herbig (1950) and Haro (1950)in observing nebulae, Herbig–Haro (HH) objects(Haro 1950;Herbig 1950) have been interpreted as optical manifestations ofpowerful outflows from newborn stars. Previously, HH objectswere studied in the ultraviolet, infrared, and radio bands to findout their morphology, hydrodynamics, and interaction withtheir surrounding media. However, many issues still need to besolved, as overviewed by Reipurth & Bally (2001). Pravdoet al. (2001) first reported X-ray emissions from HH2 at itsleading edge. After that, such X-ray emissions from protostellarjets were detected in a few HH objects(Favata et al. 2002;Bally et al. 2003; Pravdo et al. 2004, 2009; Kastner et al. 2005;Grosso et al. 2006; Linsky et al. 2007; López-Santiago et al.2015). Some jets have different X-ray properties at differentdistances from their driving sources when compared with theiroptical counterparts (Fridlund & Liseau 1998). The X-rays atthe launch site of the jet can ionize disk materials from adistance of 150au, as in the case of HH 154 (Favata et al.2006; Bonito et al. 2008), and thus significantly influence thephysical and chemical properties of the accretion disk and itsplanet-forming environment. The X-rays at large distances caninfluence the star-forming environment (Bonito et al. 2007).Hence, their physical origin attracts a lot of interest in thestellar community.

To understand the origin of X-rays in protostellar jets, manymechanisms have been proposed. For example, Bonito et al.(2004, 2007) reported that a continuous “light jet” impacting ahomogeneous ambient medium could reproduce most of thephysical properties of X-ray emission, such as temperature andemission strength. That jet had η∼10, the ratio of ambientmedium ρa to jet density ρj. Modeling with extensive spaceparameters showed that light jets could produce detectable

X-ray emission, while heavy jets could be characterized byoptical emission with very faint or no X-ray emission.Furthermore, Bonito et al. (2010) proposed that X-ray emissionwas due to multiple self-interactions of plasma blobs withdistinct velocities, ejected at different epochs by the drivingsource. Moreover, a jet/nozzle model and magnetic effects wereused to describe bright X-rays observed in HH154(Bonito et al.2011; Ustamujic et al. 2016). Recently, López-Santiago et al.(2015) suggested X-rays in HH248 were due to the impact of aheavy jet with a dense molecular cloud after passing through alow-density interstellar medium. However, the origin of X-rayemissions is still not clear because of the absence of high-resolution spectroscopy and limited observations of HH objects.Supersonic jets are also generated in experiments by intense

lasers irradiating different targets(Farley et al. 1999; Nicolaiet al. 2006; Loupias et al. 2007; Li et al. 2013, 2016; Yurchaket al. 2014; Yuan et al. 2018). Albertazzi et al. (2014) useda single, high-power laser to produce a stable and narrowcollimated outflow by applying a poloidal magnetic fieldperpendicular to the plane of a plastic target. From three-dimensional magnetohydrodynamic modeling, the hot (70 eV)region at the launching site of a plasma jet was reproducedwithout a medium, which was caused by the convergentstagnation of expanding side plasmas guided by the externalmagnetic field. This scenario completely differs from previousideas, where X-rays originate from the interaction of supersonicjets with the ambient medium. Nicolai et al. (2008, 2009)studied plasma jet interaction with gases (helium and argon)and found that the gaseous medium had a collimating effect onjet propagation(Gregory et al. 2008) and formation of the bowshock. From X-ray radiography, Dizière et al. (2015) detectedthe post-shock region with strong optical emission as a shockedambient medium with η≈0.3. However, the origin of X-rayemissions in HH objects is still ambiguous. Recently,

The Astrophysical Journal, 868:56 (7pp), 2018 November 20 https://doi.org/10.3847/1538-4357/aae83d© 2018. The American Astronomical Society. All rights reserved.

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laboratory flows were used to investigate the accretiondynamics in magnetic cataclysmic variable stars(Crosset al. 2016) and young stars with strong magnetic fields(B=20T;Revet et al. 2017).

In this work, we explore the X-ray generation of a heavy jetimpacting a denser cloud wall after passing through a low-density ambient medium, such as the case of HH248. In thisHH object, the X-ray source is far from its likely driving sourceWFC3 J05412764-0223143(López-Santiago et al. 2015). Themagnetic field is on the order of μG, which is negligiblecompared with fields near the driving source(Hartiganet al. 2007). Although an intense stellar magnetic field hasbeen suggested as the mechanism for formation and collimationof the jet, it is still unclear and is out of the scope of this paper.Here, the heavy supersonic plasma jet is generated by usingtwo convergent flows of a powerful laser facility (Shengguang-II). In Section 2, the experimental setup and theoretical modelsare outlined for hydrodynamics and spectroscopy. The resultsand discussions are presented in Sections 3 and 4. Finally,Section 5 gives conclusions of the work.

2. Methods

2.1. Experimental Setup

The experiment was performed at the Shenguang-II laserfacility in Shanghai, China. Four laser beams with a totalenergy of 920J, wavelength of λ=0.351 μm, and duration of1 ns were divided into two bunches, each consisting of two

laser beams. The experimental setup is illustrated in Figure 1.The two beam bunches synchronously irradiated two sides of aK-shaped target with an opening angle of 120° and a separationof 400 μm. Each bunch was focused to a spot with a diameterof 150 μm, which yielded an intensity of 5.3×1015W cm−2.The plasma jet was generated by two convergent supersonicflows produced by the laser irradiating the aluminum K-shapedtarget, shown in Figure 1. An obstacle of 100 μm CH film wasplaced 1.6–4mm away in the direction of jet propagation,which was affixed onto an aluminum holder.In this experiment, several diagnostic instruments were used

and configured as shown in Figure 1. Three pinhole cameraswere used to monitor the K-shaped target and the obstacleregion from different directions. Two crystal spectrometers,each with a 100 μm slit and photon energy range of1.56–1.90keV, pointed to the obstacle. An optical diagnosticsystem consisting of a shadowgraph, interferometer, andFaraday rotator was situated in the east, where the wavelengthof the probe beam was 0.53 μm with a duration of 30ps. Thisallowed measuring the electron density of plasmas between theK-shaped target and the flat obstacle.

2.2. Model—Hydrodynamics

We modeled the experimental results using a three-dimen-sional hydrodynamic simulation performed with FLASH(Fryxellet al. 2000; FLASH Userʼs Guide 2017), a well-tested adaptivemesh refinement multiphysics code. The simulation adopted anunsplit hydrosolver with a multitemperature equation of state,where the temperatures of the electron, ion, and radiation fieldswere treated separately. A multigroup diffusion approximationwas used to treat the radiative transfer and electron energyequations. Both the equation of state and opacity data werecalculated by the PrOpacEOS code (MacFarlane et al. 2006). Anon-local thermodynamic equilibrium (LTE) model was appliedto compute the atomic level populations, based on the steady-state solution of atomic rate equations. Contributions fromphotoionization and photoexcitation were neglected in this code.Boundary conditions were imposed on three directions (XYZ) tosimulate free expansion in an open space. Thermal conductivityin the model of Lee & More (1984) and viscosity were taken intoaccount. This simulation clearly described the jet generationfrom the laser irradiating the K-shaped target, as well as theinteraction between the jet and obstacle.The computational domain was set as 3500×3000×

3000 μm, adaptive mesh refinement was applied, and amaximum resolution of 7 μm was achieved. The initialconditions (such as laser and target parameters, and configura-tions) were in accordance with the experiments, where theK-shaped target was composed of Al with dimensions of500×800×800 μm and a dimple at the center with anopening angle of 120°. The CH obstacle had dimensions of112×1200×800 μm and was placed 2000 μm away.

2.3. Model—Spectroscopy

To analyze the observed X-ray spectra from the crystalspectrometer, a SASAL package (Liang et al. 2014a, 2014b) wasused to model the line emissions from the highly chargedaluminum ions to estimate the temperature of laser-producedplasmas. SASAL is a sophisticated tool for analyzing spectrosc-opy of ionization equilibrium and nonequilibrium plasmasthat includes collisional, photoionization, recombination, and

Figure 1. Schematic of the experimental setup. The four laser beams of theShengguang-II facility are arranged into two bunches, which illuminate theK-shaped Al target with an opening angle of 120°. Each beam delivers ≈240J(920 J in total) in 1 ns onto two 150 μm spots, giving an average intensity of5.3×1015 W cm−2. Three diagnostic instruments are focused on a point onthe obstacle surface, which is a 100 μm thick CH film 1.6–4mm away fromthe K-shaped target. The surface is perpendicular to the K-shaped target, whichincludes three pinhole cameras each with a beryllium window of thickness50 μm (red arrow), a crystal spectrometer (yellow), and an optical diagnosticsystem comprising a shadowgraph, a Nomarski interferometer, and a Faradayrotator (light green).

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charge-exchange processes for level and/or ionic populations.Liang et al. (2014a, 2014b) present the physical and technicaldetails and applications of this spectral package.

3. Results

López-Santiago et al. (2015) reported a heavy jet (versusinstellar medium (ISM)) impacting a denser cloud at the base ofthe Horsehead Nebula. This X-ray emission was reproducedwell by hydrodynamic FLASHmodeling (Fryxell et al. 2000)with continuous flows impacting a denser wall after passing alow-density environment. Therefore, this HH object is selectedas an astrophysical counterpart to our laboratory analog ofheavy jets. For comparison, we present a brief illustrationof HH 248 and the extended X-ray source at ∼30″ from V615Ori (05h 41m 25 7, −02° 23′ 06 0; see Figures 2(a) and 6 inLópez-Santiago et al.2015 for details).

The laboratory miniature is shown in Figures 2(b) and (c)with X-ray imaging data with time integration from the easternand western pinhole cameras. The lasers irradiating theK-shaped target generated a clear jet in dense plasma, asshown in Figure 2(c). We further observed bow-shaped X-rayemission at the surface of the obstacle along the jet propagation.This emission is much more obvious in the eastern camerabecause of the faint X-ray outflow in the small field of view(Figure 2(b)). Panels (b) and (c) also show that the laserproduced plasma, while the jet cooled without X-ray emissionbefore impacting the CH obstacle. The X-ray emissions

appeared again after impacting the CH obstacle. An unstablestructure can be clearly observed at the head of the shock frontin Figure 2(b).From an optical Nomarski interferometry image taken 3 ns

after laser irradiation, we extracted a two-dimensional electrondensity distribution by using Abel inversion as shown inFigure 2(d). The electron density between the dense outflowand obstacle is on the order of 1019cm−3. The length of theclear and irregular separatrix (nearly bow-shaped) at theobstacle is consistent with the outflow width, which meansthe interaction between them plays a role in this formation.From this panel, the velocity of a dense (above 5×1019 cm−3)outflow is estimated to be about 240±30kms−1, while thevelocity of a medium-density ((3.0±0.2)×1019 cm−3) out-flow can be up to 330±40kms−1. The errors are mainlyfrom the position uncertainty ∼50–60 μm of the plasma frontand the uncertainty (∼200 ps) of the probe beam delay time.The outflow velocity can be nearly 1000kms−1 for plasmaswith much lower density(Kuramitsu et al. 2011).Figure 3 shows the X-ray spectra in the photon energy range

1.65–1.90keV from two different spectrometers that are totallyinsensitive to C and H lines. We notice that there are two bandsof line emissions, one from the laser directly irradiating thealuminum K-shaped target and the other from the CH obstacle.There is a slight shift in the dispersion direction for the twoemission sources, which results from the angle (22°.5) betweenthe line of sight and jet propagation. In the CH obstacle region

Figure 2. Comparison between the HST/WFC3 image centered on HH 248 (Obs. ID 11548) and laboratory simulation of the ionized aluminum jet interaction. Thefront of the CH obstacle is marked by the vertical dashed line, which is also the reference position for the scale in the jet direction. The likely driving source, WFC3J05412764-0223143, of HH248 is marked by the red “+” symbol in panel (a). The path of the processed jet suggested by Frank et al. (2014) is represented by theyellow curve terminated at the extended X-ray source surrounded by a closed white curve. The X-ray images in panels (b)–(c) are from the eastern (small view-field)and western (large view-field) pinhole cameras, each with a beryllium window of thickness 50 μm. The brightness labeled in the color bar is represented by anarbitrary unit without calibration. Panel (d) is the electron density distribution resulting from Abel inversion of interferometry data taken at 3 ns after the main laserpulse.

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we observe the same emissions, Lyα and Heα, β, as those inthe K-shaped region. This clearly shows that the X-rays aroundthe CH obstacle result from the aluminum ions in thesupersonic flow, not from the impacted obstacle medium.The Lyα emission intensity at the obstacle (2 mm away fromthe target) is lower than that at the K-shaped region by a factorof 4.5; however, this factor is ∼0.6 for the He-like resonance(r). Additionally, the line ratio IHeβ/ILyα is 0.22 in the obstacleregion, being higher than that (0.12) in the target region. Fromthis line ratio, we estimated the plasma temperature to be460±16eV and 416±13eV in the K-shaped and CHregions, respectively, by using SASAL (Liang et al. 2014a,2014b).

The line intensity ratio Ii+j/Ir is also higher at theCH obstacle (0.47±0.02) than at the K-shaped target(0.32±0.02; diamond symbol in Figure 3). This ratioincreases monotonically to 0.92±0.05 at the maximumdistance of 4 mm. The flux component r decreases withincreasing distance from the CH obstacle. Electron impactexcitations following radiative decay cannot explain this lineratio at longer distances. Recombination is the main contributorto this increasing ratio, which favors the upper-level populationof i+j lines.

We further examine the effect of the obstacle material, forexample, aluminum. We notice that the line flux of the resonance rline does not change very much, but the Ii+j/Ir value of0.60±0.03 for an aluminum obstacle is smaller than0.79±0.04 for CH at an interaction distance of 3mm (see thedashed–dotted line and open circles in the bottom panel of Figure 3). This reveals that the minor component of impacted material isheated to a temperature high enough to produce He-like ions.

The hydrodynamic modeling with FLASH (Figure 4) revealsthat the X-ray emission in the K-shaped region, where theplasma cools quickly, occurs at t<0.5 ns (timing starts withthe back of the laser pulse with a duration of 1 ns). After thelaser radiation, the plasma jet starts impacting the CH obstacleat a velocity above 1000 km s−1, and the temperature increasesto 800eV at t=1 ns and then decreases slowly. The plasmadensity increases steadily in the CH obstacle region, and a bow-shaped shell forms at the surface of the obstacle. During the

evolution time of t=1–2 ns, the plasma temperature is above500eV, where the H-like aluminum ion is the dominantcontributor to observed X-ray emissions at the CH obstacle inFigures 2(b) and (c). After t=2 ns, the plasma temperature isless than 350eV, where AlXIII plays a minor role. Therefore,the observed X-ray emission at the CH obstacle results from theearlier collision at t=1–2 ns. Additionally, we notice that themorphology at t=3 ns in our FLASH model is consistent withour X-ray image (Figure 2(c)) of the dense jet and with ourinterferogram (Figure 2(d)) for the outflow density.

4. Discussion

4.1. Scaling Law for Laboratory Jet and HH248

Ryutov et al. (1999) showed that laboratory and astrophysicalplasmas will behave identically for equal rv p (where ρ is themass density and p the pressure). This is known as the “Eulersimilarity.” Our experiments and models have not taken magneticeffects into account, which would require consideration of theweak magnetic field on the order of μG in interstellar space,especially for a large-scale interaction such as in HH248. Certainassumptions are needed to make the Euler equation valid fordifferent systems, namely, that heat transport and viscosity areunimportant (meaning the Peclet number Pe?1 and Reynoldsnumber Re?1). These conditions are satisfied in Table 1. In theregion where jets are launched in the experiments, the coolingtime is t = ´ ( )T Z n1.4 10rad

22eV e ns∼3 ns with brems-

strahlung radiation loss. X-rays are clearly observed (Figure 2)and line emissions dominate the radiation loss (Figure 3). Thecooling time τ=εthm/qrad decreases to less than 250ps, whereqrad is the line emissivity and e = +( )n k T Z T3 2ithm e e isthe thermal energy density. Its radiative cooling parameter(≈vjetτrad/rjet) varies as 0.3�χ�3. In the region where jetscooled down without X-rays, the cooling time is evaluatedfrom opacity data(Drake 2006; Nicolai et al. 2008), that ist k t= + ~( ) [ ( )]Z AT f0.7 1 10mcool eV

3 ns, where A and Z arethe atomic mass and number and f (τ) a function of the opticaldepth. The hydrodynamic time is about 3 ns in the presentexperiment. The cooling parameter (thydro/tcool) will be ∼0.3. ForHH248, we found only a few references in the literature, so its

Figure 3. Left: X-ray spectra with photon energy of 1.56–1.90keV for regions in which the laser irradiates the aluminum K-shaped target (black curve) and the CHobstacle (red curve) at a distance of 2mm. Right: spectra and line intensity ratio (i+j)/r at the obstacle region for different distances are normalized by the r line at theK-shaped target. The light gray area is from our SASAL collisional calculation normalized to the observed r line intensity at electron densities of 1019–22 cm−3 andelectron temperature of 420eV.

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temperature and density are from the best-fitting parameters inLópez-Santiago et al. (2015). The impacted cloud density isabout 6×103 cm−3 at the base of the Horsehead Nebula(Ballyet al. 2018). From the photometry results in López-Santiago et al.(2015) and the guidelines for the Hubble Space Telescope WideField Camera 3,10 we used τ=3nkT V/L* to estimate thecharacteristic timescale of cooling by radiative losses to be45–70years for the E knot of HH248, where the luminosityL*=4πD2F, with D being the distance and F the line flux. Forits density we used the factor of 2–3 of the initial jet density in theFLASH model of López-Santiago et al. (2015).

Our FLASH model reveals that the CH obstacle, facing the K-shaped target, will expand toward the aluminum flow by about80 μm because of X-ray photoionization before interacting withthe jet (Figure 4). Badziak et al. (2009) reported a crater depthon a massive target of about 10 μm due to freely expandingplasma. Although there is a steep density gradient at the CHobstacle surface in this experiment, the expanded CH plasma of

depth 80 μm is enough to stagnate the aluminum jet. Therefore,an appropriate value for the electron density is about6×1019–6×1020 cm−3 for the photoionized CH plasma,which represents the denser cloud in HH248. This exceeds thelimit of our optical interferometry (4×1019 cm−3). Hence,we estimated the medium-to-jet density ratio to be 2–20 inour experiments, which is consistent with the best-fittingcontrast of 10 for the X-ray observation of HH248 reported byLópez-Santiago et al. (2015).Our analysis shows that the Euler description is valid for the

laboratory outflow and HH248, (Table 1). The variables of thetwo systems are invariant under such transformations, as L=aL1, ρ=bρ1, p=cp1, t t= a b c 1, and =v c b v1, wherea, b, and c are the coefficients of transformation betweenHH248 (with a subscript of 1) and our laser-produced plasma(no subscript). As Table 1 shows, the good scaling obtainedwith a=2×10−18, b=2.0×1017, and c=1.8×1016 andthe consistency of the Euler number ( rv p ) confirm that ourlaboratory jets can be used to investigate the origin of X-rayemissions in HH248.

Figure 4. Three-dimensional hydrodynamic modeling with FLASH code for a laser irradiating an aluminum K-shaped target at evolution times t=1 and 3 ns after themain laser pulse, as well as the outflow interaction with a CH obstacle at a distance of 0.2cm. The left panels are the electron density distributions in cm−3, while theright panels are the electron temperature distributions in eV. The spatial scaling unit of the plots is cm.

10 http://www.stsci.edu/hst/wfc3

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4.2. Origin of X-Ray Emission in HH248

López-Santiago et al. (2015) suggested that the chain ofknots in HH 248, driving source WFC J05412764-0223143,and extended X-ray source in the Horsehead Nebula follow acurved path typical of precessing jets(Frank et al. 2014). In ourexperiment, the heavy aluminum outflow passes through avacuum, then impacts the dense obstacle, which is similar tothe dense cloud wall in HH248.

Looking at morphology, we observe a similar hot X-ray“blob” with a bow-shaped front in the interaction region likethat described in Bonito et al. (2010) and López-Santiago et al.(2015) (Figure 2(b) and (c)). In best-fit modeling that includesthermal conduction(Bonito et al. 2007), the blob size is nearlydouble the initial jet radius. In our experiment, the size of thehot “blob” in the interaction region is also about double thelateral dimension of the outflow.

In models of continuous and pulsed flows(Bonito et al.2007, 2010), the surrounding dense medium decelerates lightjets by a factor of about three for detectable X-rays. In a modelwith a nozzle exit(Bonito et al. 2011), the velocity loss of thelight outflow (η=10) is at least about 450kms−1. The lostkinetic energy is converted to heat that increases thetemperature of plasma beside the shock to 8×106 K.

Efficient conversion of kinetic energy into X-ray radiation ofions (K-shell emission in our experiment) requires at leastenough kinetic energy per ion to give the plasma a hightemperature. The plasma must have additional energy both toallow for radiative losses during interaction and to excite theK/L levels efficiently. Some experiments (such as Z-pinch)have shown that a dimensionless factor x º ~K E 2i min isneeded for efficient emission, where Ki is the kinetic energy perion and Emin the minimum energy for emission. Hence, for anelement with atomic number Z and atomic mass A, the requiredvelocity for efficient K-shell emission is given by Whitneyet al. (1994) and Ampleford et al. (2014):

x x~ ~-( ) ( )v E m Z Akm s 10.0 2 10.0 2 . 11min

3.662

For the strong outflow of aluminum ions reported in this paper,the required velocity to radiate effectively in the K-shellemissions of Figures 2–3 is about 420km s−1. As discussed inSection 3, the measured outflow velocity is about 330kms−1

for plasmas with an electron density of 3.0×1019 cm−3. Thecorresponding conversion factor ξ is estimated to be 1.3, whichis an appropriate value as shown by Whitney et al. (1994). Atthe head of the outflow, plasma density is lower, but velocity ishigher. Then, the conversion factor ξ will be close to 2.Therefore, the X-ray emissions result from the thermalizationof jets due to the interaction with the surrounding medium.The work of Bonito et al. (2010) suggests that X-ray

luminosity is mainly determined by the ejection rate of plasmablobs, rather than by the jet Mach number: higher ejection ratesresult in more energetic objects, thus leading to higher X-rayluminosities. The mechanical luminosity (Ljet) of an HHoutflow is the kinetic energy supplied per second to the flowfrom the driving source, which is proportional to the mass lossaccording to p~ »˙L M r n Vjet jet jet

2jet jet (Frank et al. 2014),

where rjet, njet, and Vjet are the radius, density, and velocity ofjets, respectively. Our measurements with different distancesfor the CH obstacle reveal that the observed line intensities arerelated to the distance. FLASH modeling and optical inter-ferometry show that the outflow density decreases withincreasing distance of interaction, which means varying thedistance has effects similar to those for varying the ejectionrates. This confirms the conclusion of Bonito et al. mentionedabove.In summary, the X-ray emissions in our experiment are

consistent with observation and modeling (Bonito et al. 2010;López-Santiago et al. 2015) in terms of dynamics, morphology,ambient medium/cloud wall-to-jet contrast, and luminositydependence on ejection rate. High-resolution spectroscopy inlaboratory experiments reveals that X-rays in HH248 originatefrom jet material.

5. Conclusions

X-rays in objects such as HH248 can be understood using amodel of a heavy jet passing through the interstellar mediumand then impacting the cloud. In this work, we simulated such ajet with an intense laser facility using scaling laws for twodifferent hydrodynamic systems. We generated a supersonicoutflow with a kinetic velocity of hundreds of km s−1 thatimpacted a denser cloud as in HH 248. The observed X-rayemissions were bow-shaped, similar to the morphology foundin the modeling. High-resolution spectroscopy showed them tobe from ejected material, not from the shocked medium. Thislaboratory simulation provides a different method for under-standing stellar jet dynamics and X-ray emissions.

We acknowledge the Shenguang-II staff for operating thelaser facility. This work was supported by National Key R&DProgram of China, No. 2017YFA0402401, Science ChallengeProject, No. TZ2016005, National Natural Science Foundationof China under grants 11522326, 11622323, and BeijingNatural Science Foundation under grant 1172006, as well asKey Programs of the Chinese Academy of Sciences (QYZDJ-SSW-SLH050). We also thank Mark Kurban, M. Sc., fromLiwen Bianji, Edanz Editing China (www.liwenbianji.cn/ac),for editing the English text of a draft of this manuscript.

Table 1Comparison of Variables for Describing HH248(Bonito et al. 2008) and

Laser-produced Plasmas under the Scaling Law

Parameters Lab. Jet Scaled Lab. HH248

Te (eV) ∼40–60 1–2Average charge Z 8 1Density (cm−3) 3.0×1019 5.1×102 500±200Velocity (km s−1) 330.0 1100 �700Pressure p (bar) ∼18 10−15 10−15

Time τ ∼3ns ∼14years 35yearsJet length L 2–3mm 6700–10000au 9500–12000auViscosity (cm2 s−1) 8.7×10−4 1016 ∼1015–16

Mach number ∼6–8 ∼40–55Cooling parameter χ 0.3–3 1.3–2.0Reynold number Re ∼108 (1–6)×105

Peclet number Pe (3–5)×102 (2–10)×104

Density Ratio η 2–20a 10b

Euler number rv p 13–16 14.0–50

Notes.a Obstacle density is from present FLASH modeling; see Figure 4.b Cloud density of the Horsehead Nebula is from the paper of Ballyet al. (2018).

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ORCID iDs

G. Y. Liang https://orcid.org/0000-0003-1628-5058

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The Astrophysical Journal, 868:56 (7pp), 2018 November 20 Liang et al.