Radio‐Continuum Emission from Shocked Stellar Winds in Low‐Mass Stars

Context. Weak and compact radio continuum and H2O masers are preferred tracers of the outflow activity nearby very young stars. Aims. We want to image the centimeter free–free continuum emission in the range 1–7 cm (26–4 GHz), which arises in the inner few 1000 au from those young stars also associated with bright H2O masers. We seek to study the radio continuum properties in combination with the H2O maser kinematics to quantify the outflow energetics powered by single young stars. Methods. We made use of the Karl G. Jansky Very Large Array (VLA) in the B configuration at K band and the A configuration at both Ku and C bands in order to image the radio continuum emission toward 25 H2O maser sites with an angular resolution and thermal rms on the order of 0.′′1 and 10 μJy beam−1, respectively. These targets add to our pilot study of 11 maser sites previously presented. The sample of H2O maser sites was selected among those regions that have accurate distance measurements, obtained through maser trigonometric parallaxes, and H2O maser luminosities in excess of 10−6 L⊙. Results. We present high-resolution radio continuum images of 33 sources belonging to 25 star-forming regions. In each region, we detect radio continuum emission within a few 1000 au of the H2O masers’ position; 50% of the radio continuum sources are associated with bolometric luminosities exceeding 5 × 103 L⊙, including W33A and G240.32 + 0.07. We provide a detailed spectral index analysis for each radio continuum source, based on the integrated fluxes at each frequency, and produce spectral index maps with the multifrequency synthesis deconvolution algorithm of CASA. The radio continuum emission traces thermal bremsstrahlung in (proto)stellar winds and jets that have flux densities at 22 GHz below 3 mJy and spectral index values between − 0.1 and 1.3. We prove a strong correlation (r > 0.8) between the radio continuum luminosity (Lrad) and the H2O maser luminosity (LH2O) of (L8 GHz∕mJy kpc2) = 103.8 × (LH2O L⊙)0.74. This power-law relation is similar to that between the radio continuum and bolometric luminosities, which confirms earlier studies. Since H2O masers are excited through shocks driven by (proto)stellar winds and jets, these results provide support to the idea that the radio continuum emission around young stars is dominated by shock ionization, and this holds over several orders of magnitude of stellar luminosites (1–105 L⊙).

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Erosion of an exoplanetary atmosphere caused by stellar winds

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935543 ◽

2019 ◽

Vol 630 ◽

pp. A52 ◽

Cited By ~ 3

Author(s):

J. M. Rodríguez-Mozos ◽

A. Moya

Keyword(s):

Stellar Wind ◽

Extreme Environment ◽

Stellar Winds ◽

Habitable Zone ◽

Low Mass Stars ◽

Zone Versus ◽

Low Mass ◽

The Difference ◽

Interaction Regions ◽

The Impact

Aims. We present a formalism for a first-order estimation of the magnetosphere radius of exoplanets orbiting stars in the range from 0.08 to 1.3 M⊙. With this radius, we estimate the atmospheric surface that is not protected from stellar winds. We have analyzed this unprotected surface for the most extreme environment for exoplanets: GKM-type and very low-mass stars at the two limits of the habitable zone. The estimated unprotected surface makes it possible to define a likelihood for an exoplanet to retain its atmosphere. This function can be incorporated into the new habitability index SEPHI. Methods. Using different formulations in the literature in addition to stellar and exoplanet physical characteristics, we estimated the stellar magnetic induction, the main characteristics of the stellar wind, and the different star-planet interaction regions (sub- and super-Alfvénic, sub- and supersonic). With this information, we can estimate the radius of the exoplanet magnetopause and thus the exoplanet unprotected surface. Results. We have conducted a study of the auroral aperture angles for Earth-like exoplanets orbiting the habitable zone of its star, and found different behaviors depending on whether the star is in rotational saturated or unsaturated regimes, with angles of aperture of the auroral ring above or below 36°, respectively, and with different slopes for the linear relation between the auroral aperture angle at the inner edge of the habitable zone versus the difference between auroral aperture angles at the two boundaries of the habitable zone. When the planet is tidally locked, the unprotected angle increases dramatically to values higher than 40° with a low likelihood of keeping its atmosphere. When the impact of stellar wind is produced in the sub-Alfvénic regime, the likelihood of keeping the atmosphere is almost zero for exoplanets orbiting very close to their star, regardless of whether they are saturated or not.

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The rotation of very low-mass stars and brown dwarfs

Proceedings of the International Astronomical Union ◽

10.1017/s174392130700960x ◽

2007 ◽

Vol 3 (S243) ◽

pp. 241-248

Author(s):

Jochen Eislöffel ◽

Alexander Scholz

Keyword(s):

Angular Momentum ◽

Brown Dwarfs ◽

Magnetic Interaction ◽

Stellar Winds ◽

Solar Mass ◽

Low Mass Stars ◽

Angular Momentum Loss ◽

Rotational Evolution ◽

Solar Type ◽

Low Mass

AbstractThe evolution of angular momentum is a key to our understanding of star formation and stellar evolution. The rotational evolution of solar-mass stars is mostly controlled by magnetic interaction with the circumstellar disc and angular momentum loss through stellar winds. Major differences in the internal structure of very low-mass stars and brown dwarfs – they are believed to be fully convective throughout their lives, and thus should not operate a solar-type dynamo – may lead to major differences in the rotation and activity of these objects. Here, we report on observational studies to understand the rotational evolution of the very low-mass stars and brown dwarfs.

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The effects of stellar winds and magnetic fields on exoplanets

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314002154 ◽

2013 ◽

Vol 9 (S302) ◽

pp. 228-236 ◽

Cited By ~ 1

Author(s):

A. A. Vidotto

Keyword(s):

Solar System ◽

Interplanetary Medium ◽

Great Majority ◽

Stellar Winds ◽

Stellar Rotation ◽

Low Mass Stars ◽

Exoplanetary Systems ◽

Cool Stars ◽

Low Mass ◽

Host Stars

AbstractThe great majority of exoplanets discovered so far are orbiting cool, low-mass stars whose properties are relatively similar to the Sun. However, the stellar magnetism of these stars can be significantly different from the solar one, both in topology and intensity. In addition, due to the present-day technology used in exoplanetary searches, most of the currently known exoplanets are found orbiting at extremely close distances to their host stars (< 0.1 au). The dramatic differences in stellar magnetism and orbital radius can make the interplanetary medium of exoplanetary systems remarkably distinct from that of the Solar System. To constrain interactions between exoplanets and their host-star's magnetised winds and to characterise the interplanetary medium that surrounds exoplanets, more realistic stellar wind models, which account for factors such as stellar rotation and the complex stellar magnetic field configurations of cool stars, must be employed. Here, I briefly review the latest progress made in data-driven modelling of magnetised stellar winds. I also show that the interaction of the stellar winds with exoplanets can lead to several observable signatures, some of which that are absent in our own Solar System.

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Search for radio jets from massive young stellar objects

Astronomy and Astrophysics ◽

10.1051/0004-6361/202037652 ◽

2020 ◽

Vol 645 ◽

pp. A29

Author(s):

Ü. Kavak ◽

Á. Sánchez-Monge ◽

A. López-Sepulcre ◽

R. Cesaroni ◽

F. F. S. van der Tak ◽

...

Keyword(s):

Field Of View ◽

Radio Continuum ◽

High Mass ◽

Mass Star ◽

Low Mass Stars ◽

Star Forming ◽

Radio Jets ◽

Star Forming Regions ◽

Nonthermal Emission ◽

Low Mass

Context. Recent theoretical and observational studies debate the similarities of the formation process of high- (>8 M⊙) and low-mass stars. The formation of low-mass stars is directly associated with the presence of disks and jets. Theoretical models predict that stars with masses up to 140 M⊙ can be formed through disk-mediated accretion in disk-jet systems. According to this scenario, radio jets are expected to be common in high-mass star-forming regions. Aims. We aim to increase the number of known radio jets in high-mass star-forming regions by searching for radio-jet candidates at radio continuum wavelengths. Methods. We used the Karl G. Jansky Very Large Array (VLA) to observe 18 high-mass star-forming regions in the C band (6 cm, ≈1′′.0 resolution) and K band (1.3 cm, ≈0′′.3 resolution). We searched for radio-jet candidates by studying the association of radio continuum sources with shock activity signs (e.g., molecular outflows, extended green objects, and maser emission). Our VLA observations also targeted the 22 GHz H2O and 6.7 GHz CH3OH maser lines. Results. We have identified 146 radio continuum sources, 40 of which are located within the field of view of both images (C and K band maps). We derived the spectral index, which is consistent with thermal emission (between − 0.1 and + 2.0) for 73% of these sources. Based on the association with shock-activity signs, we identified 28 radio-jet candidates. Out of these, we identified 7 as the most probable radio jets. The radio luminosity of the radio-jet candidates is correlated with the bolometric luminosity and the outflow momentum rate. About 7–36% of the radio-jet candidates are associated with nonthermal emission. The radio-jet candidates associated with 6.7 GHz CH3OH maser emission are preferentially thermal winds and jets, while a considerable fraction of radio-jet candidates associated with H2O masers show nonthermal emission that is likely due to strong shocks. Conclusions. About 60% of the radio continuum sources detected within the field of view of our VLA images are potential radio jets. The remaining sources could be compact H II regions in their early stages of development, or radio jets for which we currently lack further evidence of shock activity. Our sample of 18 regions is divided into 8 less evolved infrared-dark regions and 10 more evolved infrared-bright regions. We found that ≈71% of the identified radio-jet candidates are located in the more evolved regions. Similarly, 25% of the less evolved regions harbor one of the most probable radio jets, while up to 50% of the more evolved regions contain one of these radio-jet candidates. This suggests that the detection of radio jets in high-mass star-forming regions is more likely in slightly more evolved regions.

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Jets in Molecular Clouds

Symposium - International Astronomical Union ◽

10.1017/s0074180900078323 ◽

1984 ◽

Vol 110 ◽

pp. 299-301

Author(s):

Arieh Königl

Keyword(s):

Molecular Clouds ◽

Direct Evidence ◽

Optical Emission ◽

Central Star ◽

Radio Continuum ◽

Low Mass Stars ◽

General Direction ◽

Low Mass ◽

Motion Measurements ◽

Infrared Sources

There is now growing evidence that the cosmic jet phenomenon manifests itself in a remarkable way in regions of active star formation embedded in dense molecular clouds. The first indications for oppositely directed, supersonic outflows from young stars were provided by molecular line observations (most notably of CO) which detected spatially separated regions of redshifted and blueshifted emission in association with embedded infrared sources. About twenty sources of this kind have been identified so far, and more are continuously being discovered; they typically have radii ∼1018 cm, velocities ∼10–50 km s−1, dynamical ages ∼104 yr, and energies ∼1046-1047 erg s−1 (see Bally and Lada 1983 for a review). Statistical arguments indicate that energetic outflows of this type are probably a common feature in stellar evolution, and that they occur in both massive and low-mass stars. Direct evidence that the outflows in many cases are highly collimated was subsequently provided by the detection of high-velocity Herbig-Haro objects (optical emission clumps with typical masses ∼10−5M⊙) along the axes of the bipolar CO lobes. Proper-motion measurements are now available for a number of these objects (e.g., Herbig and Jones 1981), and they invariably reveal that the velocity vectors (of typical magnitudes 200–400 km s−1) point away from the central star. The clumps are often found to consist of many sub-condensations which move independently with disparate speeds, but which nevertheless travel in the same general direction with an angular spread ≲ 10°. Finally, radio continuum observations (e.g., Cohen et al. 1982) and deep CCD images (e.g., Mundt and Fried 1983) have shown that the collimation of the outflows is already well established on scales of ≲ 1015 cm.

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The C:O Ratio in Dark Clouds with Cyclic Star Formation

Symposium - International Astronomical Union ◽

10.1017/s0074180900090112 ◽

1992 ◽

Vol 150 ◽

pp. 249-250

Author(s):

L A M Nejad ◽

D A Williams

Keyword(s):

Steady State ◽

Molecular Clouds ◽

Stellar Winds ◽

Low Density ◽

Low Velocity ◽

Low Mass Stars ◽

Dark Clouds ◽

Low Mass ◽

Solid Phases ◽

Phase Models

Observations (Goldsmith et al., 1986) indicate that the gas in dark clouds with embedded low-mass stars experiences a cycle, driven by stellar winds, between low and high density phases. The cycle time is sufficiently short that the chemistry never attains steady state. Icy mantles accumulating on grains during the collapse and dense core phases are thought to be removed in each cycle by low velocity shocks that terminate the low density phase. Models of this type give detailed point-by-point descriptions of both gas and solid phases in molecular clouds (Charnley et al., 1988; Nejad et al., 1990).

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A survey of circumstellar structure around young low mass stars

Symposium - International Astronomical Union ◽

10.1017/s0074180900239661 ◽

1991 ◽

Vol 147 ◽

pp. 345-352

Author(s):

S. Terebey ◽

C. A. Beichman ◽

T. N. Gautier ◽

J. J. Hester ◽

P. C. Myers ◽

...

Keyword(s):

Molecular Clouds ◽

Near Infrared ◽

Circumstellar Disk ◽

Stellar Winds ◽

Low Mass Stars ◽

Show Evidence ◽

Low Mass ◽

Circumstellar Environments ◽

Cloud Cores ◽

Infrared Array

We present results from a near-infrared array, CO interferometer, and H2O maser interferometer survey of the circumstellar environments of 26 young low-luminosity embedded stars located in nearby molecular clouds. About 75% of the sample show evidence for stellar winds/outflows in the near-infrared or CO data indicating that most of these sources are in the early wind clearing phase of their evolution. Close to 15% are multiple on the scale of 20″, suggesting that fragmentation of their surrounding dense cloud cores is important before or during gravitational collapse. Roughly 10% have H2O maser emission and the kinematics imply the masers arise in gravitationally unbound gas (i.e., a stellar wind or outflow) rather than in a circumstellar disk.

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A Molecular Disk Survey of Low-Mass Stars in the TW Hya Association

Proceedings of the International Astronomical Union ◽

10.1017/s174392131500616x ◽

2015 ◽

Vol 10 (S314) ◽

pp. 207-208

Author(s):

David R. Rodriguez ◽

Gerrit van der Plas ◽

Joel H. Kastner ◽

Adam C. Schneider ◽

Jacqueline K. Faherty ◽

...

Keyword(s):

Line Emission ◽

Mass Function ◽

Continuum Emission ◽

Molecular Gas ◽

Low Mass Stars ◽

Low Mass ◽

Cold Dust

AbstractWe have carried out an ALMA Cycle 2 survey of 15 confirmed or candidate low-mass (<0.2M⊙) members of the TW Hya Association (TWA) with the goal of detecting line emission from CO molecular gas and continuum emission from cold dust. Our targets have spectral types of M4-L0 and hence represent the extreme low end of the TWA's mass function. The survey has yielded a detection of 12CO(2–1) emission around TWA 34. This newly discovered ~10 Myr-old molecular gas disk lies just ~50pc from Earth.

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