"Cradle Of Life" Science Working Group

Welcome to the VLA2020 "Cradle of Life" wiki page. The goal of this science working group is to elaborate science cases
for a future interferometric array operating at frequencies between about 1-100 GHz.

Preliminary specifications of the array: VLANext_SpecificationsOverview_v2.pdf

Contents

A . Science cases
  1. Low mass star formation
    1.1 Probing optically thick innet regions of protostellar disks
    1.2 Binary/multiple stars
  2. High mass star formation
    2.1 Resolving the density structured and dynamics of the youngest HII regions and high-mass protostellar jets
    2.2 Star-formation around the Galactic center: A Unique Laboratory
  3. Protoplanetary disks and Planet formation
    3.1 Mapping planet forming regions in nearby disks
    3.2 Probing planet formation through the distribution of dust and pebbles
    3.3 Chemistry of planet forming regions
    3.4 Investigating disk dispersal mechanisms
  4. Planetary Science
    4.1 Deep atmosphere mapping
    4.2 Water in the upper/thin atmospheres
    4.3 Comets' chemistry
    4.3 Bistatic radar
  5. SETI
  6. Debris Disks
B . Members of the Science working group
C . 2015 AAS VLA2020 Workshop
D . Complementary material



A. SCIENCE CASES


1. LOW MASS STAR FORMATION

1.1 Probing optically thick inner regions of protostellar disks

Probing the inner regions of the disks around very young Class 0/I protostars is critical to understanding the earliest stages of disk, outflow, and jet formation. However, the inner parts of these disks may be optically thick at mm-wavelengths. If this is the case, lower frequency observations (still with very high resolution) will be necessary. There are various science drivers, including (1) determining disk masses to investigate whether self-gravitating disks form in protostars (see here and here); (2) constraining jet morphology, with the goal of potentially distinguishing between disk winds and X-winds; and (3) investigating variability at high resolution: with optically thin lines or continuum at high resolution, one could theoretically probe variability within the disk itself (which might vary independently from the protostar); furthermore, if we're probing at small spatial scales, we're also probing small timescales, which might allow us to see changes in brightness (or structure?!) on ~orbital timescales. Jets and the inner parts of disks should be free-free dominated between 1-10 GHz; we will need to observe at slightly higher frequencies to get optically thin free-free, and to get higher resolution to separate disk and jet components.

1.2 Binary/multiple stars

John Tobin's work with the VLA (VANDAM survey; see, e.g., this paper) has shown some evidence of optically thick emission in disk regions even at CARMA resolutions, suggesting that ALMA may not reveal all multiplicity. The NGVLA will allow us to probe very close binaries, closer than those Tobin is studying with the VLA: the NGVLA could push down to 5-10 AU separation at distances out to the ~1 kpc distance to Orion, allowing us to get much better statistics. While ALMA generally probes the thermal dust component of emission from disks and protostars, studies like Tobin's have shown that free-free emission is key for seeing embedded sources; by using the NGVLA, will will see the free-free component, which is another energetic diagnostic inside a collapsing envelope.

Additional notes about optical depth in disks:

A 50 AU, 0.1 M_sun disk with surface density ~ r^(-1) is optically thick (tau=1) at the following radii, per wavelength.
  • 450 micron = 73.4 AU (whole thing is thick)
  • 850 micron = 38.8 AU
  • 1.3 mm = 25.4 AU
  • 3 mm = 11 AU
  • 8mm = 4.2 AU
  • 1 cm = 3.3 AU
Assumptions: kappa_0 = 0.9 cm^2/g (dust only); M_disk = M_gas + M_dust, with a dust-to-gas ratio of 1:100; dust opacity scales as Beta=1. Note that this is for face-on disks; with any inclination, optical-depth effects will be worse.

2. HIGH MASS STAR FORMATION

Please see attached NGVLA_HMSF_slides_vX.pdf for summary slides including figures

2.1 Resolving the density structure and dynamics of the youngest HII regions and high-mass protostellar jets

The VLA/JVLA have angularly resolved and mapped the dynamics (via hydrogen ricombination lines, RLs) of the 'ultracompact' (UC) and some 'hypercompact' (HC) HII regions around (groups of) young O-type stars. Roughly, free-free emitting objects that at a distance of ~ 5 kpc have S_1.3cm ~ 100 mJy to 1 Jy, tau_1.3cm ~ 0.1 to 1, Theta_1.3cm ~ 0.03 to 0.3 pc. However, the faintest objects, those that may be the transition from a high-mass protostellar jet (shock ionized) to a (photoionized) HII region, are currently just detected in continuum and not resolved. These objects presumably are at the latest stages of accretion in the formation of stars more massive than 10 or 20 Msun.


Resolution requirement: To resolve the free-free continuum and RLs within the gravitational radius: 100 to 200 AU (for 10 to 20 Msun), or 20 to 40 mas at 5 kpc.
Max. Goal angular resolution of HPBW=6 mas at 50 GHz is enough to resolve with 2 beams the smallest radius at nu>30 GHz.

Bandwidth requirement: To do (quasi)simultaneous mapping over a large bandwidth. The smallest photoionzed HII regions are known to be variable in periods of months to years. Radio jets can be variable even within days. Multiple frequencies are needed to derive density structure. Spec. 3:1 RF bandwidth seems good enough to observe, e.g., 20 to 60 GHz or 30 to 90 GHz in a single shot.

Continuum sensitivity requirements: Scaling the calculations of Keto (2002) from 40 to 20 Msun (Phi_EUV ~ 10^48 to 10^48.5), the flux density should be S_10GHz ~ 100 muJy, S_50GHz ~ 5 mJy. Observationally, candidate trapped HC HII regions such as those in van der Tak & Menten (2005) have S_50GHz ~ 1 mJy and are not detected at lower frequencies. We take this more pessimistic flux-density estimate, and assuming that the HII region is partially-optically thick (alpha~1), and radius=20 mas, and beamFWHM_50GHz=6mas >

S_50GHz ~ 1 mJy,   beamFWHM_50GHz=6mas,   nbeams = 44  
S_25GHz ~ 0.5 mJy,   beamFWHM_25GHz=12mas,   nbeams = 11
S_12GHz ~ 0.24 mJy,   beamFWHM_12GHz=25mas,   nbeams = 2.6
S_8GHz ~ 0.16 mJy,   beamFWHM_8GHz=38mas,    nbeams = 1.1
S_5GHz ~ 0.1 mJy,   beamFWHM_5GHz=60mas,   nbeams = 0.4


Assuming further a disk of constant intensity, and an rms_10hrs = 50 nJy (rms_1hrs = 158 nJy) from 5-15 GHz and
rms_10hrs =92 nJy (rms_1hrs = 291 nJy) at higher frequencies the signal-to-noise (SN) ratios per resolution element are:

SN_50GHz_1hr = 78
SN_25GHz_1hr = 156
SN_12GHz_1hr = 584
SN_8GHz_1hr = 920


Depending on the slope of the density gradient and the size of the resolution element, the brightness towards the edges
of the HII region is lower than average. Assuming it gets down to 10% of the average.

SN_50GHz_1hr_edge = 8
SN_25GHz_1hr_edge = 16
SN_12GHz_1hr_edge = 58
SN_8GHz_1hr_edge = 92

CONCLUSION:
So the continuum of these accreting HII regions (or jet cores) can be imaged in exquisite detail with < 1 hr on source
simultaneously at all frequencies > 8 GHz.


Line sensitivity requirements: In LTE, the ratio of the line peak to continuum is ~1 at 230 GHz and this ratio scales proportionally to the frequency. Because the noise in line mode can depend a lot on the exact frequency, we use the actual frequency of typical RLs in the VLA calculator and scale down the VLA noise by x10. The result is that the x10 sensitivity is not enough to map the line at the highest possible angular resolution. Assuming a somewhat lower angular resolution, HPBW = 20 mas (2 beams per diameter in the smallest HII region, so this is a worst case scenario):

S_RL_H53a (42.95 GHz) 217 muJy, nbeams = 4, rms_20hr_5km = 15 microJy/b, SN = 3.6
S_RL_H66a (22.36 GHz) = 54 muJy, nbeams = 4, rms_20hr_5km = 11 microJy/b, SN = 1.2

So the lines can be individually mapped only in Q-band and with a significant (but feasible) investment of observing time. However, lines of similar quantum numbers can be stacked. For example, the 9 alpha-lines (H51 to H59alpha) from 30 to 50 GHz can be observed at once and stacked together. If the average SN in each map is ~2, a stacked map with SN ~ 6 can be created. Please note that this is a worst-case scenario for the smallest and faint objects.

For RLs going to higher frequencies becomes particularly relevant. For example, for H40a at 99.02 GHz, both the free-free continuum and the line-to-continuum ratio are ~x2 those of the H53a. Therefore, for that single
line and the above parameters, a mapping with SN ~ 15 could be done.


CONCLUSION:

The dynamics of these accreting HII regions (or jet cores) can be mapped with moderate signal-to-noise by either stacking cm (< 50 GHz) RLs or with individual long mm RLs (50 to 100 GHz). Note that at > 100 GHz
(i.e., ALMA) RLs can often be overwhelmed by brighter molecular lines.

2.2 Star-formation in inner few pc of the Galactic center

The VLA/JVLA have mapped the ionized mini-spirals within the Circumnuclear disk (CND) in the innner 5 pc of the Galactic
Center, Sgr A*. However, only the sensitivity and angular resolution of a NGVLA could detect the young
stellar object (YSO) population within the CND and beyond, to address the long-standing question of whether star formation
around the Galactic Center is possible, and if yes, how does it proceed.
The NGVLA could also measure the orbital motions (proper motions and line-of-sight) of the star-forming gas reservoirs within
and beyond the CND, to test scenarios of stellar migration and address how the stellar disks around Sgr A* are formed.

Frequency requirement: Ka and Q bands are ideal for avoiding the confusion either from ambient dust emission, or from the
ionized mini-spiral arms. The spectral slope from Ka to > 50 GHz bands can help diagnose the thermal emission mechanism.
This is the most obscured SF region in the Milky Way Centimeter-band observations of dust emission are highly
preferred.
The simultaneous 3:1 frequency coverage would add a lot to this science case, since the different emission mechanisms
of the possible YSOs could be imaged at once: synchrotron from stellar magnetospheres, free-free from radio jets
or stellar winds, and dust from circumstellar disks and envelopes.

Resolution requirement: 10-20 mas angular resolution, to provide 80-160 AU physical resolution, adequate to
resolve individual (proto)stellar accretion disks (if there are). High resolution observations are required to filter out the
extended confusion. The 10-20 mas resolution is adequate for the purpose of observing proper motions, just like the KECK and
VLT communities are doing with the stellar objects.

Sensitivity requirement: the estimates depend a lot on the assumption of the dust opacity. Assuming beta=1 or smaller,
the RMS of 100 nJy/beam can detect objects which have comparable mass with the Classical T Tauri disks in nearby star-forming
regions. Combining with molecular line observations (does not need very good resolution for line observations; maybe can
complement the line part with ALMA) can trace 3D orbits of individual (proto)stellar objects or gas cores.

Field of view requirement: this region is 1 to 2' wide. Dish size of the present VLA is good. Probably relatively large dishes are better
than small dishes since this would make it easier to calibrate the long baseline data at high frequency.

CONCLUSION:
- The possible in-situ star formation within the CND around the inner 2 to 4 pc from Sgr A* can be tested. The NGVLA
can detect resolved the entire putative YSO population, low- and high-mass.
- Scenarios of YSO migration can also be tested. The proper motions and line-of-sight velocities of YSOs and gas cores
within and beyond the CND can be measured. Proper motions of peculiar objects like the G2 clould could also be measured.

2.3 High resolution cm band observations for molecular line absorption against the background ionized sources

The VLA/JVLA has unambiguously diagnosed infall/accretion toward bright Ultracompact (UC) HII regions down to 100 mas angular
resolution. The NGVLA can make these diagnostics around fainter ionized sources, i.e., individual, lower-mass objects.
Note that molecular absorption against a bright background is the clearest way of diagnosing infall and accretion.
The NGVLA could also enable time-domain studies of accretion by looking for variability in the molecular absorption.

Frequency requirement: the obvious choice is NH3 lines in K and Ka bands, but there are other lines.

Sensitivity requirement: these embedded HII regions are expected to have brightness temperatures of several thousands K.
Achieving an RMS brightness temperature of ~100 K for 1 km/s velocity channel is ideal for this purpose. Lines can be stacked too.

Resolution requirement: ~20 mas is good enough to resolve the accretion flows surrounding the nearest hypercompact HII
regions (and other ionized sources) to <100 AU scale. Field of view is not an issue since these sources are typically only a few arcsec wide.

CONCLUSION:
- The NGVLA can image resolved molecular absorption against the bright (high TB) background of the ionized central regions
of massive stars in formation. This is the most unambigous way of inferring infall and accretion.
- The sensitivity and resolution of NGVLA would enable these diagnostics to be resolved in the space, velocity and time domains.

2.4 Linearly polarized dust emission in OB star- or cluster-forming cores (and synchrotron polarization)

The NGVLA would permit to understand the role of magnetic fields on the formation of massive stars and clusters,
from 20 000 AU (0.1 pc, or 3" at 6 kpc) to 100 AU scales.

- Frequency requirement: observations at 40-50 GHz are particularly useful because many objects are line crowded in the millimeter
band (e.g., Orion BN-KL), which prohibits continuum/line separation. In addition, on the ~100 AU scale, these regions are optically
thick in short millimeter band observations. Finally, long-mm wavelengths have the advantage that de-polarization due to scattering is
negligible. The cm band is also useful to observe magnetic fields related to synchrotron emission.

- Sensitivity requirement: it is difficult to predict the strength of the polarized emission. Synchtrotron polarization
has been imaged with the JVLA in a couple of landmark objects. Dust linear polarization has been detected in
envelopes, as well as in a few disks with previous mm interferometers at shorter wavelengths.

CONCLUSION:
The NGVLA could map the polarization in massive star formation regions, which may be key to halt fragmentation and
set star formation efficiencies. Polarization related to synchrotron could be done in the cm bands, whereas
dust polarization could be observed in the mm bands. The NGVLA long millimeter bands have clear advantages over
shorter-wavelength interferometers, since opacities in the smaller scales (denser regions) are low.

3. PROTOPLANETARY DISKS

3.1 Mapping planet formation regions in nearby disks

Most of the planets are predicted to form within about 10 AU from the central star where the density of the circumstellar material is the highest. Accessing these regions requires both high angular resolution and long wavelenghts observations in order to penetrate the dusty environment in which forming planets are embedded.

The beginning of ALMA operations have led to the discoveries of large holes and asymmetries in the circumstellar dust and gas distribution toward several protoplanetary disks.
These structures can be interpreted as the result of the gravitational interaction between yet unseen giant planets and the circumstellar material, and provide information on the mass and orbital radius of newborn planets. The most outstanding example is the discovery of annular gaps in the circumstellar dust distribution around the young (< 10^6 yr) low mass star HL Tau, which suggests the presence of giant planets orbiting at tens of AU from the central star. However, ALMA observations are incapable of penetrating a dust column density higher that about 1 g cm^{-2} and might be therefore provide limited informations about the innermost 5-10 AU disk regions, where the dust column density is predicted to exceed this value (for example, in a MMSN disk model, Sigma_d > 1 g cm^-2 for r < 6 AU).

By mapping the disk continuum emission on spatial scales smaller than 1 AU, the NGVLA offers a unique opportunity to investigate the distribution of dust and gas in the innermost disk regions not accessible by ALMA. NGVLA observations of the dust continuum emission at 1 and 3 cm will be able to penetrate dust layers as thick as 6 and 12 g cm^{-2}, which, in a MMSN disk, correspond to orbital radii smaller than 2 and 1 AU, respectively. Thanks to baselines 10X-20X longer than ALMA, NGVLA observations at the wavelength of 1 cm will probe spatial scales as small as 0.7 AU at the distance of 130 pc.
Finally, since the orbital time within 10 AU from the solar mass stars is less than 30 yrs, multi-epoch NGVLA observations will allow to follow the temporal evolution of planet forming disks.

The requirements to map the dust continuum emission at sub-AU resolution imply that it will be necessary to achieve a sensitivity of about 0.5 K (this corresponds to a s/n > 10 assuming a disk midplane temperature > 50 K and an optical depth of 0.1). This is captured by the preliminary NGVLA design which delivers a sensitivity of 0.32 K with 182 km baselines (i.e. 10 mas at 1 cm) and 10 hours of integration on source.
Additional notes:

- ALMA Band 1 will not achieve the required angular resolution and sensitivity: ALMA Band 1 = 100 mas at 40 Ghz --- 13 AU at 130 pc

- The dust emission is too faint at the SKA wavelengths, i.e., < 1-10 GHz

- Typical dust opacities (Beckwith & Sargent 2001):

- 2.5 cm^2 g^-1 @ 1mm - 230 GHz
- 0.8 cm^2 g^-1 @ 3 mm - 100 GHz
- 0.36 cm^2 g^-1 @ 7 mm - 40 GHz
- 0.19 cm^2 g^-1 @ 1.3 cm - 20 GHz
- 0.08 cm^2 g^-1 @ 3 cm - 10 GHz

3.2 Probing planet formation through the distribution of dust and pebbles

Recent millimeter-wave observations of protoplanetary disks have revealed that the spatial distribution of circumstellar dust is shaped by the aerodynamic interactions between dust and gas, which strongly depends on the dust grain size. In particular, grains smaller than a few microns are coupled to the gas, while millimeter dust grains and larger are concentrated toward gas pressure maxima. This process can lead local enhancements on the dust-to-gas ratio with strong implications on the formation of planetesimals (see, e.g., the review by Johansen et al. 2014 for PPVI).

Before planet formation occurs, large dust grains migrate toward the innermost disk regions where they interact to form planetesimals. However, as soon as the first planets form, they gravitationally interact with the circumstellar gas producing local enhancements in the gas pressure which can catalyze the formation of planetesimals at much larger orbital radii. This process is currently observed toward transitional disks and might have played an important role in forming trans-Neptunian objects and comets in our Solar system.

The NGVLA will allow to map the distribution of pebbles across the entire planet formation process. The comparison between ALMA and NGVLA observations will allow us to investigate the evolutions of solids and the formation of planetesimals.

wenfu_gas.png wenfu_dust.png wenfu_dust_1cm.png
Snapshot of the gas and dust surface density of a disk perturbed by a 10 MJ planet orbiting at 20 AU from the central star, after 600 orbits of the planet. The planet is locate at (0,-20).
Left: Gas surface density. A circular gap and a vortex develop at the orbital radius of the planet and at the outer edge of the gap, respectively. Center: Surface density of 1 mm grains. Millimeter grains are concentrated toward the center of the vortex, where the dust-to-gas ratio reaches values close to 1, and in the Lagrangian points on the planet orbit. Right: Surface density of 1 cm grains. Centimeter grains are even more concentrated toward the center of the vortex (modified from Fu et al. 2014).

3.3 Chemistry of planet forming regions (modified from the disk SKA chapter, Testi et al. 2015)

How far the chemistry of complex organic molecules and pre-biotic molecules goes in the interstellar medium is an important area to be understood in order to figure out the complexity of the material that is delivered on the planetary surfaces. There is convincing evidence for the presence of simple amminoacids in meteorites and possibly in comets of our own Solar System (Pizzarello et al. 1991; Glavin et al. 2006; Elsila et al. 2009). Laboratory experiments have also shown that complex molecules and simple amminoacids like Glycine (NH2CH2COOH) can form in irradiated interstellar ice analogs (Muñoz Caro et al. 2002; Holtom et al. 2005; Modica & Palumbo 2010). The recent detection of water vapour in the prestellar core L1544 shows that even in the cold interior of these dense cores, cosmic rays yelding secondary high energy radiation can desorb a measurable quantity of molecules from the ices (Caselli et al. 2012). Jiménez-Serra et al. (2014) have shown that not only water, but also complex pre-biotic molecules (like Glycine) may be detectable if desorbed together with the water molecules.

We expect that this result can be extended to the cold midplanes of protoplanetary disks. The direct detection in the gas phase and measurements of the abundance with respect to water of complex organic molecules would be a significant milestone in studying our cosmic heritage and the ability of ISM and disks chemistry to produce the raw material required for the development of life on exoplanets. As complex organic molecules are easily destroyed in the warm molecular layers of disks, which are exposed to the heating radiation from the star, the best hope to detect and study them is in the cold interior of disks, where they may be desorbed from ices as outlined. In these cool conditions, the rotational spectra of complex molecules is significantly skewed towards low frequencies.

The low frequencies of the NGVLA (compared to ALMA) gives access to
1. NH3, one of the most (perhaps the most) important N-bearing volatile,
2. volatile molecules in very dense regions, e.g. the innermost parts of protoplanetary disks, that are otherwise veiled by optically thick dust at shorter wavelengths,
3. low-lying excitation modes of moderately complex organic molecules, enabling the observations of molecules such as CH3CN in the cold regions where they are proposed to form through ice chemistry, and
4. lines of very complex organic molecules such as glycine because of a lower line density at cm wavelengths compared to mm wavelengths.

NH3: NH3 is a major carrier of nitrogen in interstellar and perhaps circumstellar environments. It is also the most commonly used gas thermometer. NH3 is frequently observed in pre- and protostellar regions, but lack of sensitivity has prevented its detection in protoplanetary disk. Obtaining accurate gas temperatures in disks is a key goal of disk explorations since it regulates both the chemistry and physics, including planet migration patterns. The importance of nitrogen for prebiotic chemistry also makes it disturbing that we currently do not have observational access to any of its main carriers during planet formation. If sensitive enough, NGVLA could address both these important science topics.

Astrochemical explorations of high opacity regions: With the advent of high-spatial resolution and high-sensitivity molecular line mm-observations with ALMA it is becoming clear that a major obstacle to exploring the disk midplane of the innermost 10s of AU of disks is dust optical depth. This can only be resolved by high-spatial resolution observations a longer wavelengths where the dust opacities are smaller. Since this region is the planet forming region in most disks, characterizing its volatiles is an important objective. Accessible NGVLA lines include NH3 (see above), H2CO 1-1, 2-2, 3-3, CH3OH 1-0, and perhaps OH, HCO2+, HDO and H2O if NGVLA is designed with sufficient sensitivity.

Cold organic molecules: Most observations of complex organic molecules have focused on hot regions where thermal desorption of icy grain mantles bring all volatiles into the gas-phase. These molecules form cold, however, and to trace the chemical evolution requires observations of low-lying excitation states of the small abundances of organic molecules that desorb non-thermally in their formation zones. The cm and 7mm spectral regions give access to these transitions for all common complex organic molecules, including CH3CN.

Detections of glycine and other complex organics: Very complex organics are difficult to detect for two reasons. One they are less abundant and therefore require high sensitivity, and two at these low signal strengths the mm spectra are crowded with emission from complex molecules and their isotopologues. At longer wavelengths the spectra are less crowded, enabling their detections if the sensitivity is present.

Also the updated molecular line list is:

- NH3 1-1, 2-2, 3-3 etc and the deuterated equivalents
- OH at 1.66736 GHz (perhaps best way to estimate water in PPDs)
- H2O at 22.23508 GHz to trace hot water
- HDO at 10.27825 GHz
- HCO2+ (i.e. protonated CO2 and perhaps the best tracer or CO2) 1-0 (21.38315 GHz) and 2-1 (42.76620 GHz)
- Lines of very complex organic molecules (COMs) including glycine 3-2 ladder at 19-22 GHz, 4-3 ladder at 26-28 GHz, 5-4 ladder at 32-37 GHz etc
- Low-excitation lines of less complex COMs, including CH3OH 1-0 at 47 GHz and H2CO 1-1, 2-2, 3-3 and 4-4 transitions and CH3CN 1-0 at 18.39778 GHz

3.4 Investigating disk dispersal mechanisms (modified from the disk SKA chapter, Testi et al. 2015)

Young stellar objects with disks produce high energy radiation in the ultraviolet and X-rays, caused by the accretion of material from the disk to the young star and by the active chromospheres. X-ray variability is thought to originate from flaring powered by reconnection events in the stellar magnetosphere and is known to be very variable in young stellar objects with disks (see e.g. the recent surveys in ONC, Taurus and Ophiuchus star forming regions: Grosso et al. 2005; Güdel et al. 2007; Pillitteri et al. 2010). The high energy radiation from the central star (either from accretion or chromospheric activity) is known to affect disk evolution (Gorti et al. 2009), and is also expected to affect disk gas and solids chemistry (eg. Ábrahám et al. 2009; Banzatti et al. 2012). A systematic study of the effect of this high-energy irradiation variability on the disks is lacking because of the difficulties to set up coordinated X-ray and millimetre/infrared campaigns. Radio continuum observations in the 1-12 GHz range is an important tool to study the long term effects of stellar radiation on the disk by constraining the photoevaporative winds (Galván- Madrid et al. 2014; Pascucci et al. 2011, 2012), and has also the potential of allowing us to execute coordinated studies for the effects of energetic flares on the disk chemistry. X-ray activity and flaring is also known to be connected with radio flaring, although the sensitivity of existing radio facilities is limited and only a fraction of the X-ray detected objects are also identified as radio flaring (eg. Forbrich et al. 2011; Forbrich & Wolk 2013).

To detect and characterize photoevaporative winds in nearby star forming regions, it is necessary to reach μJy sensitivities at 1-10 GHz (Pascucci et al. 2012; Galván-Madrid et al. 2014) at least over the areas of the nearby star forming
regions in the Gould Belt (these requirements are well captured by preliminary NGVLA design which delivers a sensitivity of 50 nJy between 5-15 GHz in 10 hours of integration on source).

One possible important avenue to pursue these studies in the future will be through the resolved spatial and kinematical imaging of the HI radio recombination lines expected in the disk photoevaporative flows (Pascucci et al. 2012). To allow resolved imaging, high sensitivity and imaging performance at 0.1′′ angular resolution and 0.1 km/s spectral resolution are required.

4. PLANETARY/EXOPLANETARY SCIENCE

Note by B. Butler: I have attached to this page the summary chapter on planetary science from the 2004 SKA book (edited by Chris Carilli & Steve Rawlings) which i co-wrote with Imke, Don Campbell, and Dale Gary (for the solar part). In the book this is pages 1511-1536. Some of it is outdated, but much is still valid. There were also chapters in that book on spacecraft tracking (written by Dayton Jones) and astrobiology/SETI (written by Jill Tarter) that are probably still pretty valid as well.

4.1 Deep atmosphere mapping (continuum)

By observing at all frequencies (0.3 – 100 GHz), which probe different depths in the atmospheres, one can determine the 3D spatial distribution of absorbers in the planets’ deep atmospheres. This is an excellent method to investigate deep atmospheric dynamics (e.g., see de Pater et al., 2014a). When observing at specific wavelengths simultaneously, the horizontal and vertical temperature field might be extracted from the data.
  • Need for Improved spatial resolution of maps.
Why important: All giant planets have dynamically maintained compositional gradients, storms, and other features at all length scales. Identifying these features, and how they interact, is fundamental to our understanding of the dynamics and energy flow within the atmosphere. Why the current VLA is insufficient: For Neptune and Uranus (2.2" and 3.6" diameter), VLA does not have the spatial resolution to resolve anything but the largest-scale features in the troposphere (tropospheric studies rely on wavelengths > 1 cm). The NGVLA can do it: Longer baselines, increased collecting area, increased bandwidth.

  • Need for Improved temporal resolution of mapping (i.e. full-disk imaging on time scales < 1 hour), allowing us to resolve longitudinal features in the atmosphere.
Why important: Longitudinal features, such as Jupiter's Great Red Spot or the "hot spot" near Neptune's pole (Orton et al. 2011, de Pater et al., 2014a), offer important clues to atmospheric composition, structure, and dynamics. Imaging at the widest range of wavelengths possible (from visible to meter wavelengths) helps us understand what these features are and what their vertical extent is. Why the current VLA is insufficient: Most mapping of the giant planets averages over many hours, to increase sensitivity and maximize uv coverage. Since these planets rotate with periods of 10 to 17 hours, however, longitudinal features are lost. The one exception to date is recent work on Jupiter (Sault et al. 2004; de Pater et al. 2014b), but the techniques employed may not work on the much fainter giant planets further away. Why the NGVLA can do this: Increased number of baselines. Increased collecting area. Increased bandwidth.

  • Need for simultaneous mapping at high and low spatial resolution, i.e., the need for short spacing data.
To analyze/simulate the data with radiative transfer models, knowledge of the absolute brightness temperatures is a must. Due to temporal variability, the planets must simultaneously be observed at all spatial scales. This cannot be done with the VLA; it is imperative it can be done with the NGVLA.

4.2 Water in upper/thin atmospheres

4.3 Comets' chemistry and nuclei

The direct observation of of ammonia in comets is very difficult because it is a short-lived molecule with only 5000 s life-time at 1 AU from the Sun and the lines are very weak. So the high spatial resolution and sensitivity provided by NGVLA are essential for mapping both water and ammonia lines in the near-nucleus cometary coma. Possible experiments include:
  • True ammonia/water ratio: can be derived by simultaneous mapping of 22 GHz water line and 24 GHz ammonia line in the near-nucleus coma (within 1000 km). Understanding the true ammonia abundance relative to water should enable us to explore how comets link back to the protoplanetary disk. Ammonia observations are also proposed as a science case for disk studies with NGVLA. These two cases are closely connected and relevant.
  • Testing the ammonia/water ratio heterogeneity in the coma and nucleus through the cometary jet activity near the nucleus surface.
Observations of cometary nuclei can be directly compared with observing asteroids, albeit that one probes much deeper in a comet because of its mainly icy composition. Bistatic radar of comets (see below) provides information on the nuclei and the “icy grain halo”.

4.4 Bistatic Radar

Radar observations have been used to probe the surfaces of almost all bodies in the solar system with solid surfaces. Notable findings include the indications of water ice in the permanently shadowed regions at the poles of the Moon and Mercury. More recently, there has been considerable interest in using radar observations both to characterize near-Earth objects (NEOs) and determine their orbits precisely. In particular, the orbits determined from radar observations are sufficiently precise that they can be used to assess whether a NEO presents any risk of colliding with the Earth over the next several decades to a century.

Radar observations are currently conducted in the S band (~ 2.3 GHz) and X band (~ 8 GHz). Future radar observations may also be conducted in the Ka band (~ 32 GHz). All of these bands could be within the frequency coverage of the NGVLA. The NGVLA could serve as the receiving element for bistatic experiments. Bistatic radar observations are particularly useful for NEOs with close approaches to the Earth, during which it can be difficult or impossible to switch the radar facility from transmitting to receiving. Further, the signal-to-noise ratio of radar observations scales as R^{-4}, where R is the range to the object. The increased sensitivity of the NGVLA would increase the range to which NEOs could be targeted for radar observations.

4.5 Subsurfaces' thermal emission

NGVLA wavelength sound the thermal emission from subsurfaces and surfaces:
  • Sounding of depth in the 3-60 cm range: possibility to derive the vertical temperature profile within and under the diurnal thermal skin (section of surface sensitive to diurnal variations)
  • High resolution mapping of Mars, Venus, Mercury (all frequencies), large moons and Pluto (mid and high frequencies), main-belt asteroids down to the 100-km size (high frequencies)
  • Direct indication of thermal and radiative properties of the subsurface material: dielectric constant, porosity, roughness, thermal inertia
  • Direct access to dielectric constant through polarization measurements.

4.6 Brown dwarfs cyclotron emission

4.7 Jupiter synchtron

Jupiter’s synchrotron radiation is changing over time in response to e.g., comet impacts, interplanetary shocks and any other phenomenae that may induce changes in the energetic electron distribution in Jupiter’s magnetosphere. With the signal-to-noise in the older VLA maps, and changes in Jupiter’s size as the planet orbits the Sun, it is difficult to assess the reality of short-term time variations. With the NGVLA, and the ability to observe at short and long spacings simultaneously, such studies can be carried out. Most value is obtained by observing over the entire frequency range, from the lowest frequencies (0.3 GHz) up to ~50 GHz. If time variations are observed, the source and mode of transport of energetic electrons can be determined, one of the outstanding questions in jovian magnetospheric physics.

4.8 Rings

Much information can be extracted from radio observations of Saturn’s rings. High precision maps of the rings at high spatial resolution at different frequencies (2-50 GHz) can be used to determine the mass fraction of non-icy material in the rings and how this varies throughout the rings, as well as the nature and size distribution of the smallest particles in Saturn’s rings (Zhang et al., 2014). As for atmospheric observations of the giant planets, also these ring observations require simultaneous coverage of small and long baselines. Measuring the polarization of the rings is also useful complementary quantity to determine particle properties from their scattering characteristics.

5. SETI

SETI (Search for ExtraTerrestrial Intelligence) experiments seek to determine the distribution of advanced life in the universe through detection of emission from advanced technology. For more than 100 years, technology constructed by human beings has been producing radio emission that would be readily detectable at 10s of pc using receiving technology only moderately more advanced than our own. Some emission, including that produced by the planetary radars at Arecibo Observatory and the NASA Deep Space Network, would be detectable across our galaxy. Modern radio SETI experiments have been ongoing for the last 55 years, but for the most part they have searched only a small fraction of the radio spectrum accessible from the surface of the Earth and have probed only a few nearby stars at high sensitivity. The sensitivity and flexibility of the NGVLA, leveraged by the wealth of new SETI targeting information that will be forthcoming from experiments such as GAIA and TESS, could enable a vastly more powerful radio SETI search than has been conducted in the past. For example, at 5 x EVLA sensitivity, NGVLA could detect radio emission of comparable luminosity to our own high-power aircraft radar from dozens of nearby stars in only 10 minutes from 100s of MHz through to K band. Further, the proposed frequency coverage and putative sky coverage of the NGVLA would be very complementary to other upcoming radio SETI facilities, including the SKA.

SETI observations require access to low level time-domain data products in order to achieve high sensitivity to coherent and/or technologically modulated signals. For interferometers, generally this is accomplished in a manner similar to operating the telescope as a single unified VLBI station - e.g. the signals from all antennas are phased up (or beamformed) to one or more positions on the sky and the resultant voltage time series is recorded to disk or analyzed in-situ with one or more custom signal processing systems. Multiple beams on the sky are especially effective because they allow powerful multi-beam interference excision to be employed in the candidate sorting stage. Although SETI signal processing systems are custom in the sense that they run specific signal detection algorithms that may differ from those used in other radio astronomy applications, recently the hardware portion has been largely constructed from commodity elements, e.g. rack mount CPUs and GPUs.

One could envision several possible paradigms for SETI observations with the NGVLA, including both commensal and primary-user modes. In the case of the former, SETI observations would be most successful if a capability were built into the overall system that permitted simultaneous beamforming, and subsequent SETI signal analysis, within the primary beam of the telescope regardless of what other activities were underway. In general, SETI targets are isotropic on the sky. Although one can make reasonable arguments for concentrating on regions of high galactic stellar density in raster scan surveys, there are sufficient unknowns that a true all-sky survey is at least as effective. For both commensal and primary-user modes, access to as wide a phased bandwidth as possible will maximize search speed.

6. Debris Disks

Mapping the distribution of solids in debris disks is important to:
1. identify signposts of dynamical interaction between Kuiper/asteroid belts and planetary systems, and indirectly infer the presence of planets in a parameter space (low masses, large semi-major axes) that is not accessible to other observational techniques.
2. investigate extra-solar asteroid and Kuiper belts.

While observations at infrared wavelengths are sensitive to micron-sized grains which strongly interact with the radiation field of the central star, millimeter-centimeter wave observations probe larger pebbles which are virtually unaffected by it and probe the location and dynamics of the large parent bodies (asteroids or Kuiper belt-like objects).
Because of their faint fluxes relative to the younger primordial disks, so far the detection of debris disks in dust continuum emission at lambda ~ 7mm or longer has been reported for very few disks only (Wilson et al. 2011, Ricci et al. 2012). The better sensitivity of the NGVLA relative to the other current radio telescopes will certainly improve on it. However, because of the steep decline of dust emission with wavelength, ALMA will probe the structure of small solids at a better signal-to-noise ratio than the NGVLA. (Note that ALMA wavelengths are long enough to probe particles that are not significantly affected by the stellar radiation field, and dust emission is optically thin because of the low surface densities).

Combining NGVLA and ALMA observations will be interesting to probe the spectral index of the dust emission which is related to the shape of the solid size distribution and can provide useful tests to the collisional models of Kuiper belts (Ricci et al. 2012, Pan & Schlichting 2012).

As for gas observations, few debris disks have been detected in atomic/molecular lines at any wavelength. No gas detection has been reported at radio wavelengths so far and it is not clear whether the NGVLA will be sensitive enough to be useful in this regard.

B. MEMBERS OF THE SWG GROUPED INTO SCIENTIFIC SUB-AREAS

The name in boldface are the coordinators for each sub-area.

LOW MASS STAR FORMATION

Chat Hull
Doug Johnstone
John Tobin
Laurent Loinard
James di Francesco
Paola Caselli
Tyler Bourke

HIGH MASS STAR FORMATION

Roberto Galvan-Madrid
Peter Schilke
Qizhou Zhang
Betsy Mills
Crystal Brogan
Hauyu Baobab Liu
Steve Longmore
Henrik Beuther
Alvaro Sanchez-Monge
Adam Ginsburg
Maite Beltran

PROTOPLANETARY DISKS AND PLANET FORMATION

Andrea Isella
Laura Perez
Claire Chandler
Leonardo Testi
Sean Andrews
Jonathan Williams
John Monnier (liason with the Planet Formation Imager project http://planetformationimager.org)
Thomas Hennings
---- overlap with the astrochemistry group ----
Ted Bergin
Karin Oberg
Charlie Qi

DEBRIS DISKS

Luca Ricci
David Wilner
John Carpenter
Mark Wyatt

ASTRO CHEMISTRY/ASTRO BIOLOGY

Karin Oberg
Ted Bergin
Geoff Blake
Paola Caselli
Charlie Qi
Betsy Mills
Maite Beltran

PLANETARY/EXOPLANETARY SCIENCE

Arielle Moullet
Bryan Butler
David Alexander
Joseph Lazio
Raphael Moreno
Imke de Pater
Mark Hofstadter
Charlie Qi
Andrew Siemion


C. 2015 AAS VLA2020 Workshop

The activities of the CoL science working group will be presented on January 4th, 2015 at the VLA2020 workshop held in Seattle.

- Summary of the NGVLA science cases (Andrea Isella)

- TBD (Meredith Hughes)

- TBD (Brenda Matthews)

- TBD (Lorant Loindard)

- discussion

D. Complementary material

North American Array White Paper: http://science.nrao.edu/A2010/rfi/PPP-NAA-edited.pdf
EVLA Memo 91 (SKA high):http://www.aoc.nrao.edu/evla/geninfo/memoseries/evlamemo91.pdf
ALMA Band 1 Science Case: http://arxiv.org/abs/0910.1609
Recent SKA Conference Summary: http://arxiv.org/abs/1408.5317
SKA science talks from Sicily: http://astronomers.skatelescope.org/documents/aaska14-presentations/
Cradle of Life SKA WG: https://indico.skatelescope.org/conferenceDisplay.py?confId=266

-- ArielleMoullet - 2014-10-10

Last edited by:

-- BryanButler - 2014-12-04
Topic attachments
I Attachment Action Size Date Who Comment
NGVLA_HMSF_slides_v1.pdfpdf NGVLA_HMSF_slides_v1.pdf manage 1405.4 K 2014-11-28 - 21:52 RobertoGalvanMadrid (Possible) slides for the High-Mass Star Formation subgroup
NGVLA_HMSF_slides_v2.pdfpdf NGVLA_HMSF_slides_v2.pdf manage 281.7 K 2014-12-19 - 14:55 RobertoGalvanMadrid Part 2 of (possible) slides for the High-Mass Star Formation subgroup
VLANext_SpecificationsOverview_v2.pdfpdf VLANext_SpecificationsOverview_v2.pdf manage 71.1 K 2014-10-23 - 09:56 ArielleMoullet  
butler-et-al-ska.pdfpdf butler-et-al-ska.pdf manage 885.9 K 2014-12-04 - 13:50 BryanButler Butler et al. 2004 SKA book chapter on planetary science with SKA. Some parts outdated, but much of it is still valid.
wenfu_dust.pngpng wenfu_dust.png manage 374.6 K 2014-12-02 - 10:30 ArielleMoullet  
wenfu_dust_1cm.pngpng wenfu_dust_1cm.png manage 278.1 K 2014-12-02 - 10:30 ArielleMoullet  
wenfu_gas.pngpng wenfu_gas.png manage 451.6 K 2014-12-02 - 10:27 ArielleMoullet gas surface density
Topic revision: r55 - 2014-12-22, ArielleMoullet
 

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