Solar-mass Protostar Research Articles

An Observational View of Structure in Protostellar Systems

The envelopes and disks that surround protostars reflect the initial conditions of star and planet formation and govern the assembly of stellar masses. Characterizing these structures requires observations that span the near-IR to centimeter wavelengths. Consequently, the past two decades have seen progress driven by numerous advances in observational facilities across this spectrum, including the Spitzer Space Telescope, Herschel Space Observatory, the Atacama Large Millimeter/submillimeter Array, and a host of other ground-based interferometers and single-dish radio telescopes. ▪Nearly all protostars have well-formed circumstellar disks that are likely to be rotationally supported; the ability to detect a disk around a protostar is more a question of spatial resolution rather than whether or not a disk is present.▪The disks around protostars have inherently higher millimeter/submillimeter luminosities as compared to disks around more-evolved pre-main-sequence stars, though there may be systematic variations between star-forming regions.▪The envelopes around protostars are inherently asymmetric, and streamers emphasize that mass flow through the envelopes to the disks may not be homogeneous.▪The current mass distribution of protostars may be impacted by selection bias given that it is skewed toward solar-mass protostars, which is inconsistent with the stellar initial mass function.

Early Planet Formation in Embedded Disks (eDisk). X. Compact Disks, Extended Infall, and a Fossil Outburst in the Class I Oph IRS43 Binary

We present the first results from the Early Planet Formation in Embedded Disks Atacama Large Millimeter/submillimeter Array Large Program toward Oph IRS43, a binary system of solar mass protostars. The 1.3 mm dust continuum observations resolve a compact disk, ∼6 au radius, around the northern component and show that the disk around the southern component is even smaller, ≲3 au. CO, 13CO, and C18O maps reveal a large cavity in a low-mass envelope that shows kinematic signatures of rotation and infall extending out to ∼2000 au. An expanding CO bubble centered on the extrapolated location of the source ∼130 yr ago suggests a recent outburst. Despite the small size of the disks, the overall picture is of a remarkably large and dynamically active region.

The Chemical Nature of Orion Protostars: Are ORANGES Different from PEACHES? ORANGES II.

Abstract Understanding the chemical past of our Sun and how life appeared on Earth is no mean feat. The best strategy we can adopt is to study newborn stars located in an environment similar to the one in which our Sun was born and assess their chemical content. In particular, hot corinos are prime targets because recent studies have shown correlations between interstellar complex organic molecules abundances from hot corinos and comets. The ORion ALMA New GEneration Survey aims to assess the number of hot corinos in the closest and best analog to our Sun’s birth environment, the OMC-2/3 filament. In this context, we investigated the chemical nature of 19 solar-mass protostars and found that 26% of our sample sources show warm methanol emission indicative of hot corinos. Compared to the Perseus low-mass star-forming region, where the PErseus ALMA CHEmistry Survey detected hot corinos in ∼60% of the sources, the hot corinos seem to be relatively scarce in the OMC-2/3 filament. While this suggests that the chemical nature of protostars in Orion and Perseus is different, improved statistics is needed in order to consolidate this result. If the two regions are truly different, this would indicate that the environment is likely playing a role in shaping the chemical composition of protostars.

Evolution of CAI-sized Particles during FU Orionis Outbursts. I. Particle Trajectories in Protoplanetary Disks with Beta Cooling

Abstract Solar-type young stellar objects undergo periodic, energetic outbursts that appear to be the result of enhanced mass accretion driven by the gravitational instability of their disks. Such FU Orionis outbursts may have profound consequences for the earliest solids in a protoplanetary disk, namely the refractory inclusions containing abundant calcium and aluminum (CAIs). We present models of the orbital evolution of centimeter-radius particles representing large CAIs in marginally gravitationally unstable disks. The hydrodynamical evolution of the disks is calculated with a fully three-dimensional code, including compressional heating and cooling in the beta cooling approximation. The particles are initially distributed uniformly throughout the disk, which extends from 1 to 10 au around a solar-mass protostar, but within ∼100 yr the particles are concentrated by gas drag into regions surrounding the spiral arms and rings formed by the gas disk. The particles settle down toward the disk midplane, only to be lofted repeatedly upward by shock fronts. Large-scale radial transport both outward and inward occurs, with significant numbers of particles reaching the outer disk (∼10 au) and surviving for considerably longer times than would be the case in a quiescent disk with gas pressure monotonically decreasing with distance from the protostar. Individual particles experience wide ranges of disk temperatures during their journeys, ranging from 60 K in the outer disk to nearly 2000 K in spiral features. Future work will consider the implications for CAI rims of the thermochemical processing experienced during FU Orionis outbursts.

Knotty protostellar jets as a signature of episodic protostellar accretion?

Aims. We aim to study the causal link between the knotty jet structure in CARMA 7, a young Class 0 protostar in the Serpens South cluster, and episodic accretion in young protostellar disks. Methods. We used numerical hydrodynamics simulations to derive the protostellar accretion history in gravitationally unstable disks around solar-mass protostars. We compared the time spacing between luminosity bursts Δτmod, caused by dense clumps spiralling on the protostar, with the differences of dynamical timescales between the knots Δτobs in CARMA 7. Results. We found that the time spacing between the bursts have a bi-modal distribution caused by isolated and clustered luminosity bursts. The former are characterized by long quiescent periods between the bursts with Δτmod = a few × (103–104) yr, whereas the latter occur in small groups with time spacing between the bursts Δτmod = a few × (10–102) yr. For the clustered bursts, the distribution of Δτmod in our models can be fit reasonably well to the distribution of Δτobs in the protostellar jet of CARMA 7, if a certain correction for the (yet unknown) inclination angle with respect to the line of sight is applied. The Kolmogorov–Smirnov test on the model and observational data sets suggests the best-fit values for the inclination angles of 55–80°, which become narrower (75–80°) if only strong luminosity bursts are considered. The dynamical timescales of the knots in the jet of CARMA 7 are too short for a meaningful comparison with the long time spacings between isolated bursts in our models. Moreover, the exact sequences of time spacings between the luminosity bursts in our models and knots in the jet of CARMA 7 were found difficult to match. Conclusions. Given the short time that has passed since the presumed luminosity bursts (tens to hundreds years), a possible overabundance of the gas-phase CO in the envelope of CARMA 7 compared to what could be expected from the current luminosity may be used to confirm the burst nature of this object. More sophisticated numerical models and observational data on jets with longer dynamical timescales are needed to further explore the possible causal link between luminosity bursts and knotty jets.

Water and acetaldehyde in HH212: The first hot corino in Orion

Aims: Using the unprecedented combination of high resolution and sensitivity offered by ALMA, we aim to investigate whether and how hot corinos, circumstellar disks, and ejected gas are related in young solar-mass protostars. Methods: We observed CH$_3$CHO and deuterated water (HDO) high-excitation ($E_{\rm u}$ up to 335 K) lines towards the Sun-like protostar HH212--MM1. Results: For the first time, we have obtained images of CH$_3$CHO and HDO emission in the inner $\simeq$ 100 AU of HH212. The multifrequency line analysis allows us to contrain the density ($\geq$ 10$^{7}$ cm$^{-3}$), temperature ($\simeq$ 100 K), and CH$_3$CHO abundance ($\simeq$ 0.2--2 $\times$ 10$^{-9}$) of the emitting region. The HDO profile is asymmetric at low velocities ($\leq$ 2 km s$^{-1}$ from $V_{\rm sys}$). If the HDO line is optically thick, this points to an extremely small ($\sim$ 20--40 AU) and dense ($\ge$ 10$^{9}$ cm$^{-3}$) emitting region. Conclusions: We report the first detection of a hot corino in Orion. The HDO asymmetric profile indicates a contribution of outflowing gas from the compact central region, possibly associated with a dense disk wind.

THE FORMATION OF MASSIVE PRIMORDIAL STARS IN THE PRESENCE OF MODERATE UV BACKGROUNDS

Radiative feedback from populations II stars played a vital role in early structure formation. Particularly, photons below the Lyman limit can escape the star forming regions and produce a background ultraviolet (UV) flux which consequently may influence the pristine halos far away from the radiation sources. These photons can quench the formation of molecular hydrogen by photo-detachment of $\rm H^{-}$. In this study, we explore the impact of such UV radiation on fragmentation in massive primordial halos of a few times $\rm 10^{7}$~M${_\odot}$. To accomplish this goal, we perform high resolution cosmological simulations for two distinct halos and vary the strength of the impinging background UV field in units of $\rm J_{21}$. We further make use of sink particles to follow the evolution for 10,000 years after reaching the maximum refinement level. No vigorous fragmentation is observed in UV illuminated halos while the accretion rate changes according to the thermal properties. Our findings show that a few 100-10, 000 solar mass protostars are formed when halos are irradiated by $\rm J_{21}=10-500$ at $\rm z>10$ and suggest a strong relation between the strength of UV flux and mass of a protostar. This mode of star formation is quite different from minihalos, as higher accretion rates of about $\rm 0.01-0.1$ M$_{\odot}$/yr are observed by the end of our simulations. The resulting massive stars are the potential cradles for the formation of intermediate mass black holes at earlier cosmic times and contribute to the formation of a global X-ray background.

CH in absorption in IRAS 16293-2422

While recent studies of the solar-mass protostar IRAS16293-2422 have focused on its inner arcsecond, the wealth of Herschel/HIFI data has shown that the structure of the outer envelope and of the transition region to the more diffuse ISM is not clearly constrained. We use rotational ground-state transitions of CH (methylidyne), as a tracer of the lower-density envelope. Assuming LTE, we perform a $\chi^2$ minimization of the high spectral resolution HIFI observations of the CH transitions at ~532 and ~536 GHz in order to derive column densities in the envelope and in the foreground cloud. We obtain column densities of (7.7$\pm$0.2)$\times10^{13}$ cm$^{-2}$ and (1.5$\pm$0.3)$\times10^{13}$ cm$^{-2}$, respectively. The chemical modeling predicts column densities of (0.5-2)$\times10^{13}$ cm$^{-2}$ in the envelope (depending on the cosmic-ray ionization rate), and 5$\times10^{11}$ to 2.5$\times10^{14}$ cm$^{-2}$ in the foreground cloud (depending on time). Both observed abundances are reproduced by the model at a satisfactory level. The constraints set by these observations on the physical conditions in the foreground cloud are however weak. Furthermore, the CH abundance in the envelope is strongly affected by the rate coefficient of the reaction H+CH$\rightarrow$C+H$_2$ ; further investigation of its value at low temperature would be necessary to facilitate the comparison between the model and the observations.

Deep observations of O2toward a low-mass protostar withHerschel-HIFI

According to traditional gas-phase chemical models, O2 should be abundant in molecular clouds, but until recently, attempts to detect interstellar O2 line emission with ground- and space-based observatories have failed. Following the multi-line detections of O2 with low abundances in the Orion and rho Oph A molecular clouds with Herschel, it is important to investigate other environments, and we here quantify the O2 abundance near a solar-mass protostar. Observations of O2, at 487 GHz toward a deeply embedded low-mass Class 0 protostar, NGC 1333-IRAS 4A, are presented, using the HIFI instrument on the Herschel Space Observatory. Complementary data of the chemically related NO and CO molecules are obtained as well. The high spectral resolution data are analysed using radiative transfer models to infer column densities and abundances, and are tested directly against full gas-grain chemical models. The deep HIFI spectrum fails to show O2 at the velocity of the dense protostellar envelope, implying one of the lowest abundance upper limits of O2/H2 at <6x10^-9 (3 sigma). However, a tentative (4.5 sigma) detection of O2 is seen at the velocity of the surrounding NGC 1333 molecular cloud, shifted by 1 km/s relative to the protostar. For the protostellar envelope, pure gas-phase models and gas-grain chemical models require a long pre-collapse phase (~0.7-1x10^6 years), during which atomic and molecular oxygen are frozen out onto dust grains and fully converted to H2O, to avoid overproduction of O2 in the dense envelope. The same model also reproduces the limits on the chemically related NO molecule. The tentative detection of O2 in the surrounding cloud is consistent with a low-density PDR model with small changes in reaction rates. The low O2 abundance in the collapsing envelope around a low-mass protostar suggests that the gas and ice entering protoplanetary disks is very poor in O2.

Fine and hyperfine structure of the transition of ND() in vibrational excited states

The deuterated radical ND was produced in a DC discharge cell cooled at liquid nitrogen temperature. The discharge proved to be vibrationally hot, therefore the transient species could be detected in its vibrational excited states up to v = 6. By scanning in the 431–531 GHz frequency region, several fine-structure components of the transition in vibrational excited states were observed, each of them showing a complex hyperfine structure. A global analysis, including the measured frequencies and previous submillimetre-wave and infrared data, allowed an accurate determination of the equilibrium spectroscopic parameters of the ND radical including fine and hyperfine constants. A very precise determination of the equilibrium bond length r e was obtained. This value is not consistent with the value reported in the literature from NH data. This incongruity was discussed in terms of the breakdown of the Born–Oppenheimer approximation. In view of the recent detection of ND in a solar-mass protostar [A. Bacmann et al., Astron. Astrophys. 521, L42 (2010)], an extended spectroscopic characterization of this deuterated isotopologue of the NH species may prove useful, considering the large deuterium enhancement observed in molecular clouds.

EVOLUTION OF THE SOLAR NEBULA. IX. GRADIENTS IN THE SPATIAL HETEROGENEITY OF THE SHORT-LIVED RADIOISOTOPES60Fe AND26Al AND THE STABLE OXYGEN ISOTOPES

Short-lived radioisotopes (SLRI) such as $^{60}$Fe and $^{26}$Al were likely injected into the solar nebula in a spatially and temporally heterogeneous manner. Marginally gravitationally unstable (MGU) disks, of the type required to form gas giant planets, are capable of rapid homogenization of isotopic heterogeneity as well as of rapid radial transport of dust grains and gases throughout a protoplanetary disk. Two different types of new models of a MGU disk in orbit around a solar-mass protostar are presented. The first set has variations in the number of terms in the spherical harmonic solution for the gravitational potential, effectively studying the effect of varying the spatial resolution of the gravitational torques responsible for MGU disk evolution. The second set explores the effects of varying the initial minimum value of the Toomre $Q$ stability parameter, from values of 1.4 to 2.5, i.e., toward increasingly less unstable disks. The new models show that the basic results are largely independent of both sets of variations. MGU disk models robustly result in rapid mixing of initially highly heterogeneous distributions of SLRIs to levels of $\sim$ 10% in both the inner ($<$ 5 AU) and outer ($>$ 10 AU) disk regions, and to even lower levels ($\sim$ 2%) in intermediate regions, where gravitational torques are most effective at mixing. These gradients should have cosmochemical implications for the distribution of SLRIs and stable oxygen isotopes contained in planetesimals (e.g., comets) formed in the giant planet region ($\sim$ 5 to $\sim$ 10 AU) compared to those formed elsewhere.

GIANT PLANET FORMATION BY DISK INSTABILITY IN LOW MASS DISKS?

Forming giant planets by disk instability requires a gaseous disk that is massive enough to become gravitationally unstable and able to cool fast enough for self-gravitating clumps to form and survive. Models with simplified disk cooling have shown the critical importance of the ratio of the cooling to the orbital timescales. Uncertainties about the proper value of this ratio can be sidestepped by including radiative transfer. Three-dimensional radiative hydrodynamics models of a disk with a mass of $0.043 M_\odot$ from 4 to 20 AU in orbit around a $1 M_\odot$ protostar show that disk instabilities are considerably less successful in producing self-gravitating clumps than in a disk with twice this mass. The results are sensitive to the assumed initial outer disk ($T_o$) temperatures. Models with $T_o$ = 20 K are able to form a single self-gravitating clump, whereas models with $T_o$ = 25 K form clumps that are not quite self-gravitating. These models imply that disk instability requires a disk with a mass of at least $\sim 0.043 M_\odot$ inside 20 AU in order to form giant planets around solar-mass protostars with realistic disk cooling rates and outer disk temperatures. Lower mass disks around solar-mass protostars must rely upon core accretion to form inner giant planets.

The dependence of the IMF on the density- temperature relation of pre-stellar gas

AbstractIt has been recently shown by several authors that fragmentation of pre-stellar gas (i.e. at densities from 104 to 1010 particles cm−3 and temperatures of order 10-30 K) depends on the gas thermodynamics much more than it was anticipated in earlier studies, in which only an isothermal behaviour has been assumed for the gas. Here we review the results of a number of numerical hydrodynamic simulations (e.g. Li et al. 2003, Jappsen et al. 2005, Bonnell et al. 2006) in which departure from isothermality has been attempted by employing a polytropic equation of state (eos) with exponent different from unity. In particular, in these studies it has been shown that the dominant fragmentation scale of pre-stellar gas, and hence the peak of the initial mass function (IMF), depends on a polytropic exponent that changes value, from below to above unity, at a critical density (Larson 2005). Furthermore, this piecewise polytropic eos depends on the gas metallicity and fundamental constants. Therefore, the peak of the IMF depends, in turn, also on the gas metallicity and fundamental constants rather than on initial conditions, as it has been previously suggested (e.g. Larson 1995). Hence, we are for the first time in a position to infer theoretically the notion of a universal IMF (at least for its low-mass end).We also present two test cases in which a non-isothermal eos has been used in the context of smoothed particle hydrodynamic (SPH) numerical simulations. In the first case star formation is triggered by means of low-mass clump collisions. These calculations have shown that clump collisions can be a relatively efficient mechanism for the formation of solar-mass protostars and their lower-mass companions (efficiency greater or of order 20-25%; Kitsionas &amp; Whitworth 2006). We have also found that in such collisions protostars form mainly by fragmentation of dense filaments along which it is likely that pairs of protostars capture each other in close binaries surrounded by circumbinary discs. In the second case, the use of a polytropic eos with a varying exponent appropriate for the metallicity of starburst regions (Spaans &amp; Silk 2000, 2005) is shown to be sufficient to obtain a top heavy IMF similar to that observed e.g. in the Galactic centre (Klessen, Spaans &amp; Jappsen 2006). These are preliminary results in the direction of revisiting earlier isothermal calculations that were resolving all densities up to the opacity limit for fragmentation (e.g. Bate et al. 2002ab, 2003), this time also taking into account the thermal properties of the gas in the density range between 104 and 1010 particles cm−3. The next step would be to include self-consistent radiation transport in the calculations, the first attempts for which are already in the making (e.g. Whitehouse &amp; Bate 2004).

Spitzerand Magellan Observations of NGC 2264: A Remarkable Star‐forming Core near IRS 2

We analyze {\it Spitzer} and Magellan observations of a star forming core near IRS-2 in the young cluster NGC 2264. The submillimeter source IRAS 12 S1, previously believed to be an intermediate mass Class 0 object is shown to be a dense collection of embedded, low mass stars. We argue that this group of stars represents the fragmenting collapse of a dense, turbulent core, based on a number of indicators of extreme youth. With reasonable estimates for the velocity dispersion in the group, we estimate a dynamical lifetime of only a few x 10$^{4}$ years. Spectral energy distributions of stars in the core are consistent with Class I or Class 0 assignments. We present observations of an extensive system of molecular hydrogen emission knots. The luminosity of the objects in the core region are consistent with roughly solar mass protostars.

Organic molecules in protoplanetary disks around T Tauri and Herbig Ae stars

The results of single-dish observations of low- and high-J transitions of selected molecules from protoplanetary disks around two T Tauri stars (LkCa 15 and TW Hya) and two Herbig Ae stars (HD 163296 and MWC 480) are reported. Simple molecules such as CO, CO, HCO+, CN and HCN are detected. Several lines of H2CO are found toward the T Tauri star LkCa 15 but not in other objects. No CH3OH has been detected down to abundances of 10-9–10-8 with respect to H2. SO and CS lines have been searched for without success. Line ratios indicate that the molecular emission arises from dense (10–10 cm) and moderately warm ( 20–40 K) intermediate height regions of the disk atmosphere between the midplane and the upper layer, in accordance with predictions from models of the chemistry in disks. The sizes of the disks were estimated from model fits to the 12CO 3–2 line profiles. The abundances of most species are lower than in the envelope around the solar-mass protostar IRAS 16293-2422. Freeze-out in the cold midplane and photodissociation by stellar and interstellar ultraviolet photons in the upper layers are likely causes of the depletion. CN is strongly detected in all disks, and the CN/HCN abundance ratio toward the Herbig Ae stars is even higher than that found in galactic photon-dominated regions, testifying to the importance of photodissociation by radiation from the central object in the upper layers. DCO+ is detected toward TW Hya, but not in other objects. The high inferred DCO+/HCO+ ratio of ~0.035 is consistent with models of the deuterium fractionation in disks which include strong depletion of CO. The inferred ionization fraction in the intermediate height regions as deduced from HCO+ is at least 10-11–10-10, comparable to that derived for the midplane from recent H2D+ observations. Comparison with the abundances found in cometary comae is made.

Collapse and expansion in the bright-rimmed cloud SFO 11NE

We report the results of a search for the double-peaked blue-skewed infall signature in the bright-rimmed cloud core SFO 11NE SMM1. Observations of the optically thick HCO$^{+}$ and optically thin H$^{13}$CO$^{+}$ J=3--2 lines reveal that there is indeed a characteristic double-peaked line profile, but skewed to the red rather than the blue. Modelling of the dust continuum emission and line profiles show that the motions within SFO 11NE SMM1 are consistent with a collapsing central core surrounded by an expanding outer envelope. We show that the collapse is occurring at a similar rate to that expected onto a single solar-mass protostar and is unlikely to represent the large-scale collapse of gas onto the infrared cluster seen at the heart of SFO 11NE SMM1. The outer envelope is expanding at a much greater rate than that expected for a photoevaporated flow from the cloud surface. The modelled expansion is consistent with the bulk cloud re-expansion phase predicted by radiative-driven implosion models of cometary clouds.

The mass profile in the solar-mass stars

It does seem clear that, the solar-mass stars are born from molecular clouds. We have no well-confirmed observation of a solar-mass protostar. However, the mass distribution in a star, mass of the core, and mass of the dark interstellar cloud from which the star was formed as well as the other characteristics related to the early stages of the star formation are predicted by the Energy Transfer Model. Two main assumed sources of energy for the solar-mass stars formation are originated from the gravitational pressure and thermonuclear reactions. The model was tested for the nearest us star, the Sun. The results are in good agreement with the Standard Solar Model.

Star Formation from Turbulent Fragmentation

Star formation is intimately linked to the dynamical evolution of molecular clouds. Turbulent fragmentation determines where and when protostellar cores form, and how they contract and grow in mass via accretion from the surrounding cloud material. Using numerical models of self-gravitating supersonic turbulence, efficiency, spatial distribution and timescale of star formation in turbulent interstellar clouds are estimated. Turbulence that is not continuously replenished or that is driven on large scales leads to a rapid formation of stars in a clustered mode, whereas interstellar turbulence that carries most energy on small scales results in isolated star formation with low efficiency. The clump mass spectrum for models of pure hydrodynamic turbulence is steeper than the observed one, but gets close to it when gravity is included. The mass spectrum of dense cores is log-normal for decaying and large-wavelength turbulence, similar to the IMF, but is too flat in the case of small-scale turbulence. The three-dimensional models of molecular cloud fragmentation can be combined with dynamical pre-main sequence stellar evolution calculations to obtain a consistent description of all phases of the star formation process. First results are reported for a one solar mass protostar.

Formation of planetary systems and brown dwarfs around single stars

The conditions for the formation of planets and brown dwarfs around single main-sequence stars are considered in two scenarios. The formation of planets and brown dwarfs requires that the initial specific angular momentum of a solar-mass protostar be (0.32)×1018 cm2/s. The accreted matter of the protostar envelope forms a compact gas ring (disk) around the young star. If the viscosity of the matter in this ring (disk) is small, increasing its mass above a certain limit results in gravitational instability and the formation of a brown dwarf. If the viscosity of the gas is sufficiently large, the bulk of the protostar envelope material will be accreted by the young star, and the gas disk will grow considerably to the size of a protoplanetary dust disk due to the conservation of angular momentum. The formation of dust in the cool part of the extended disk and its subsequent collisional coalescence ultimately results in the formation of solar-type planetary systems.

Dynamical Collapse of Nonrotating Magnetic Molecular Cloud Cores: Evolution through Point‐Mass Formation

We present a numerical simulation of the dynamical collapse of a nonrotating magnetic molecular cloud core and follow the core's evolution through the formation of a central point mass and its subsequent growth to a 1 solar-mass protostar. The epoch of point-mass formation (PMF) is investigated by a self- consistent extension of previously presented models of core formation and contraction in axisymmetric, self-gravitating, isothermal, magnetically supported interstellar molecular clouds. Prior to PMF, the core is dynamically contracting and is not well approximated by a quasistatic equilibrium model. Ambipolar diffusion, which plays a key role in the early evolution of the core, is unimportant during the dynamical pre-PMF collapse phase. However, the appearance of a central mass, through its effect on the gravitational field in the inner core regions, leads to a revitalization of ambipolar diffusion in the weakly ionized gas surrounding the central protostar. This process is so efficient that it leads to a decoupling of the field from the matter and results in an outward propagating hydromagnetic C-type shock. The existence of an ambipolar diffusion-mediated shock was predicted by Li & McKee (1996), and we find that the basic shock structure given by their analytical model is well reproduced by our more accurate numerical results. Our calculation also demonstrates that ambipolar diffusion, rather than Ohmic diffusivity operating in the innermost core region, is the main field decoupling mechanism responsible for driving the shock after PMF.