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<jats:title>ABSTRACT</jats:title> <jats:p>We have used the Karl G. Jansky Very Large Array (VLA) to carry out multi-epoch radio continuum monitoring of the Orion Nebula Cluster (ONC) and the background Orion Molecular Cloud (OMC; 3 epochs at Q band and 11 epochs at Ka band). Our new observations reveal the presence of 19 radio sources, mainly concentrated in the Trapezium Cluster and the Orion Hot Core (OHC) regions. With the exception of the Becklin–Neugebauer object and source C (which we identify here as dust emission associated with a proplyd) the sources all show radio variability between the different epochs. We have found tentative evidence of variability in the emission from the massive object related to source I. Our observations also confirm radio flux density variations of a factor &gt;2 on timescales of hours to days in five sources. One of these flaring sources, OHC-E, has been detected for the first time. We conclude that the radio emission can be attributed to two different components: (i) highly variable (flaring) non-thermal radio gyrosynchrotron emission produced by electrons accelerated in the magnetospheres of pre-main-sequence low-mass stars and (ii) thermal emission due to free–free radiation from ionized gas and/or heated dust around embedded massive objects and proplyds. Combining our sample with other radio monitoring at 8.3 GHz and the X-ray catalog provided by <jats:italic>Chandra</jats:italic>, we have studied the properties of the entire sample of radio/X-ray stars in the ONC/OMC region (51 sources). We have found several hints of a relation between the X-ray activity and the mechanisms responsible for (at least some fraction of) the radio emission. We have estimated a radio flaring rate of ∼0.14 flares day<jats:sup>−1</jats:sup> in the dense stellar cluster embedded in the OHC region. This suggests that radio flares are more common events during the first stages of stellar evolution than previously thought. The advent of improved sensitivity with the new VLA and ALMA will dramatically increase the number of stars in young clusters detected at radio wavelengths, which will help us improve our understanding of the origin and nature of the radio emission.</jats:p>

<jats:title>ABSTRACT</jats:title> <jats:p>Ionizing stellar photons heat the upper regions of planetary atmospheres, driving atmospheric mass loss. Gas escaping from several hot, hydrogen-rich planets has been detected using UV and X-ray transmission spectroscopy. Because these planets are tidally locked, and thus asymmetrically irradiated, escaping gas is unlikely to be spherically symmetric. In this paper, we focus on the effects of asymmetric heating on local outflow structure. We use the Athena code for hydrodynamics to produce 3D simulations of hot Jupiter mass loss that jointly model wind launching and stellar heating via photoionization. Our fiducial planet is an inflated, hot Jupiter with radius <jats:inline-formula> <jats:tex-math> <?CDATA ${R}_{{\rm{p}}}=2.14{R}_{\mathrm{Jup}}$?> </jats:tex-math> <?MML <mml:math> <mml:msub> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">p</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>2.14</mml:mn> <mml:msub> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>Jup</mml:mi> </mml:mrow> </mml:msub> </mml:math>?> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apj516680ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> and mass <jats:inline-formula> <jats:tex-math> <?CDATA ${M}_{{\rm{p}}}=0.53{M}_{\mathrm{Jup}}$?> </jats:tex-math> <?MML <mml:math> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">p</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>0.53</mml:mn> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>Jup</mml:mi> </mml:mrow> </mml:msub> </mml:math>?> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apj516680ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>. We irradiate the initially neutral, atomic hydrogen atmosphere with 13.6 eV photons and compute the outflow’s ionization structure. There are clear asymmetries in the atmospheric outflow, including a neutral shadow on the planet’s nightside. Given an incident ionizing UV flux comparable to that of the Sun, we find a steady-state mass loss rate of <jats:inline-formula> <jats:tex-math> <?CDATA $\sim 2\times {10}^{10}$?> </jats:tex-math> <?MML <mml:math> <mml:mspace width="-0.16em"/> <mml:mspace width="-0.16em"/> <mml:mo>&sim;</mml:mo> <mml:mn>2</mml:mn> <mml:mo>&times;</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> </mml:msup> </mml:math>?> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apj516680ieqn3.gif" xlink:type="simple" /> </jats:inline-formula> g s<jats:sup>−1</jats:sup>. The total mass loss rate and the outflow substructure along the substellar ray show good agreement with earlier 1D models, for two different fluxes. Our 3D data cube can be used to generate the outflow’s extinction spectrum during transit. As a proof of concept, we find absorption of stellar Ly<jats:italic>α</jats:italic> at Doppler-shifted velocities of up to ±50 km s<jats:sup>−1</jats:sup>. Our work provides a starting point for further 3D models that can be used to predict observable signatures of hot Jupiter mass loss.</jats:p>