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Stellar evolution and the Sun

Millimeter continuum emission from stars

Over the last two decades, the high sensitivity and spatial resolution of the VLA has revolutionized stellar radio astronomy. Using the VLA, major advances were made in studies of non-thermal radiation from a wide variety of stellar types. Recent upgrades in the VLA bandwidth have increased sensitivity, and this revolution has continued. ALMA has extended imaging of stars to higher frequencies.

Photospheric emission becomes much easier to observe at millimeter wavelengths owing to the Rayleigh-Jeans Law. ALMA can resolve the photospheres and chromospheres of giant and supergiant stars within a few hundred parsecs. Moreover, in addition to free-free emission, ALMA will allow (sub)millimeter imaging of thermal emission from dust in stellar envelopes. ALMA will detect the photospheres of stars across the HR diagram, including those in the Bright Star Catalog, as it did recently for α Centauri (Liseau et al A&A 573, L4 2015).

ALMA’s ability to detect the photospheres of so many stars allows it to measure positions relatively often to astrometric accuracy. The orbit of any planet around its central star causes that star to undergo a reflexive circular motion around the star-planet barycenter. By taking advantage of the incredibly high resolution of ALMA in its widest configuration, we may be able to detect this motion. This will enable ALMA to indirectly detect planets which may orbit these stars.

Asymptotic Giant Branch Stars

As nuclear fuel deep within a star is burned, instabilities occur. These may result in rapid increase in heating, which may be seen by observers as a thermal pulse driven episode of mass loss. Nuclear-processing enriches the outer envelopes of the star; as dense warm material enriched in C, N, O, P, S and Si deep in the star cools, condensation produces stardust. The dust is accelerated outward during pulses and drags gas along with it into the circumstellar envelope, which may be imaged in dust continuum or in the lines of molecules such as CO, SiO and their isotopic variants. Thus, ALMA provides a sensitive probe of not only the physics of the stellar matter but of its nucleosynthetic origins. In several Early Science studies, ALMA’s high resolution has identified unknown components of stellar systems while its spectral line sensitivity has provided insight into isotopic variations within lost-mass shells and into dust formation processes near the stellar photosphere.

In one remarkable recent study, ALMA has imaged the detached molecular shell and circumstellar medium of the AGB star R Sculptoris with unprecedented detail during Cycle 0 (Maercker et al 2012). The ALMA data shows that the shell originates with change in mass loss rate and expansion velocity during a stellar thermal pulse. An ALMA image in a line of CO at 0.9mm wavelength reveals spiral structure in shell, suggesting an unseen companion star or massive planet modulates the loss of mass from the star. The ALMA observation demonstrates change in mass loss rate by a factor of thirty accompanied by a decrease in the expansion velocity.

Further observations (Vlemmings et al 2013 A&AP 556, L1) found variations in the 12CO/13CO intensity ratios in R Scl, signifying abundance variation in the isotopes. The ratio in the shell (~19) differs from that (>60) near the star; the ratio varies by more than a factor of ten within the shell. Isotopic fractionation in the cool (~35K) shell along with selective dissociation, perhaps owing to an embedded ultraviolet radiation source, may account for these variations although some part of them may arise from variation owing to nuclear processing in the star.

Decin et al. (2015A&A 574,5) showed that correlated structures seen in molecular images of the evolved carbon star CW Leo (IRC+10216) may result from spiral shell structure induced by an unseen binary companion, in a similar fashion to that seen in R Scl.

 

CW Leo

Figure 1. CW Leo spectra from ALMA in selected parts of a 20 GHz bandwidth measured by ALMA. Intensity is in Jy/bm and spectral resolution 1 MHz. Upper panels (red) show IRAM 30m spectra; lower panes the ALMA data.

Cernicharo et al (2013 ApJ 778 L25) imaged IRC+10216 with ALMA in nine excited vibrational states of HNC J=3-2 covering energies up to 5300 K. The radius of the unresolved emitting region is 0.6" or three stellar radii, similar to the event of dusty clumps observed in the infrared. The physical and chemical conditions in the dust formation zone should be characterized by modeling of inner envelope species. Owing to the high sensitivity, a briar patch of narrow and unidentified lines was seen.

 

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Figure 2. Image of CO in the J=3-2 line from ALMA Cycle 0 data. The blue spiral connects emission components and shows the pattern of mass loss from the central star guided into the spiral by an unseen companion.

 

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Figure 3. The Boomerang Nebula reveals its true shape with ALMA. The background blue structure, as seen in visible light (HST), shows a classic double-lobe shape with a very narrow central region. ALMA’s ability to see the cold molecular CO gas reveals the nebula’s more elongated shape, in contours. Contours levels shown are at 5s (white), 10s (blue), 20s (magenta), 30s (gray), 40s (red), and 50s (black), with s = 7.5 mJy/beam. The beam (FWHM) is shown as the white ellipse in the lower left corner. The ALMA data validate suggest a model incorporating two nested spherically symmetric shells: a warm inner shell extending 2.5–6'' with an expansion velocity of about 35 km/s and a cool, extended outer shell extending 6''–33'', with a velocity of about 164 km/s. The latter shell is cooled below the temperature of the cosmic microwave background through adiabatic expansion. The ALMA observations show that the inner component is bipolar, with a dense waist, and the outer component is patchy but roughly circular and similar in dimensions to the model, bearing in mind that a significant fraction of the flux in absorption has been resolved out. Credit: Bill Saxton; NRAO/AUI/NSF; NASA/Hubble; Raghvendra Sahai.

 

Post-AGB stars.

After the outer shell of the star is lost, the hot central stellar remnant is exposed, and its energetic radiation begins to ionize the remnant circumstellar shell, creating a planetary nebula. Many planetaries are markedly asymmetric, with material flowing outward at high velocity in the polar direction, and at lower velocities equatorially. Often a region of neutral gas and dust is found in the equatorial plane; ALMA provides excellent high spatial and kinematic sensitivity for imaging these regions. The Boomerang Nebula is one example of such an object. An ALMA image of the Boomerang obtained by Sahai, Vlemmings, Huggins, et al. (ApJ 777, 92) shows a central hourglass-shaped pre-planetary nebula surrounded by a patchy, but roughly round, cold high-velocity outflow centered on a dense waist containing large grains (yellow in the image above). As the flow from the central star expands, it cools through adiabatic expansion. This process has cooled the envelope substantially below the temperature of the Cosmic Microwave Background, the 2.7K relic radiation from the Big Bang which pervades the Universe. Thus the region appears to be among the coldest in the Universe, ironic for material so close to a hot central star. The ALMA observations also show that the outer regions of the CO flow are rewarmed, probably by photoelectric grain heating.

Supergiant and Hypergiant Stars

The Supergiant star VY Canis Majoris

An oxygen-rich red supergiant well-known for the complexity and large mass of its envelope is VY CMa. Using publicly available ALMA Science Verification data, Richards et al. (2014) and O’Gorman et al. (2015) imaged VY CMa with in the submm dust continuum and in the emission lines of water masers at 658, 321, and 325 GHz. The ALMA images at the highest frequency trace dust on spatial scales down to 11 R (71AU), locating two prominent components. One of these is associated with the star but the brighter, component ‘C’, lying about 400 AU to the southeast is massive, cool (Td ≲ 100 K) and lacks molecular emission near its peak. Overall the dust emission displays an anisotropic morphology, with 17% of the total dust mass located in clumps within a roughly spherical stellar wind. This pattern suggests continuous directional mass loss over decades. The masers extend over ~90 km s-1 and lie in irregular thick shells about the center of expansion, confirmed as the stellar position. Different transitions, however, form non-overlapping clumps with velocities consistent with expansion but exhibiting drastic and irregular departures from spherical symmetry. The maser inner rims lie at successively larger distances from the star, while the outer rims lie within the central 1.2” at ~2500, 6300, and 7500 AU in the order of the transitions given above. Bright elongated maser features suggestive of shocks are seen at up to 2000AU from the star, perhaps associated with dust formation.

The Hypergiant star &eta Carinae

The object &eta Carinae is considered to be an important short-lived unstable phase in the life of the most massive stars in galaxies, shortly before they explode as supernovae. It is one of the most luminous galactic sources and is regarded as an extreme case of a Luminous Blue Variable. This southern source may consist of two very massive (50-80 M) stars in a wide 5.5 year orbit. The binary is surrounded by a highly bipolar nebula containing 1-2 M of material which was ejected by one of the stars in 1843. No other stellar object (apart from supernovae) is known to have such extreme mass loss (i.e., ≈ 2x10−3Myr−1). At submillimeter and millimeter wavelengths, &eta Carinae is also avery strong source showing regular variations with a cycle of 5.5 years (it reaches about 40 Jy at maximum at 230 GHz). The variation in flux density is due to eclipsing events from the binary system. In addition, &eta Carinae is a strong emitter in the hydrogen

Supernovae

Millimeter observations of supernovae (SN) offer exciting possibilities, complementing observations at other wavelengths in important ways. As the radio spectrum of a supernova evolves, the higher peak flux density is expected to occur earlier in the millimeter wavelength regime. Thus at millimeter wavelengths one might observe supernovae more promptly and further away than at longer wavelengths. Very bright SNII could easily be detected andmonitored with ALMA up to a distance of 350 Mpc, i.e. z ~ 0.1.

 

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Figure 4. ALMA identified CO and SiO in the SN1987A inner ejecta. The CO clumps comtain at least 0.1Msun of 12CO, an order of magnitude more than measured in the first few years after the explosion.

 

As the ejecta expand and cool, dust is formed. Recent ALMA observations of Supernova 1987A in the Large Magellanic Cloud showed that dust continues to form. Furthermore, in the central regions CO and SiO molecules are also found. In fact, although the data were too fragmentary for an analysis, both abundant Si isotopes, 28Si and 29Si were imaged over partial velocity extents. This suggests that ALMA might probe the nucleosynthetic character of the debris around the remnant, illuminating the evolution of its central star. ALMA views the full velocity range of emission, unobscured by dust. Doppler tomography will be possible with the completed ALMA in CO and other molecules that will probe the spatial, chemical and kinetic environment within the inner ejecta. [Kamenetzky et al., 2013 ApJ, 773, 34.]

Gamma ray bursts

Gamma ray bursts (GRBs) are the brightest transient phenomena in the Universe, arising from poorly understood physical catastrophes which may involve the collapse of the densest core of a massive star to a Black Hole, or possibly the coalescence of two collapsed objects. Millimeter emission lags the high energy burst in time and can provide key insight into the development of the burst. The development of the burst can then be used to derive the energy, density and other key parameters of the explosion and help characterize the nature of the originating event. Even pre-Early Science ALMA was capable of measuring a GRB—test observations were made of GRB110715A at z= 0.8224 and made public as part of Science Verification (Sanchez-Ramirez, de Ugarte Postigo, Gorosabel et al. 2013, Highlights of Spanish Astrophysics VII, 399).

The Sun

ALMA constitutes the largest ground-based instrument for solar research built in our decade. At a recent Workshop, attendees noted that ALMA will address several key solar physics problems posing particular observational challenges:

– Imaging the dynamic chromosphere: one of the greatest challenges to ALMA since it will entail imaging a source that fills the beam and varies on a time scale of tens of seconds.

– Imaging solar flares: A recent mystery has revolved around the so-called "sub-THz" spectral component to certain large flares. Unlike the expected synchrotron component, it shows an inverted spectrum.

– Radio Recombination Lines: High-n hydrogen RRLs have been detected in the 350 and 450 micron bands using an FTS. It is expected that certain ions (e.g., O VI) may also be detected as the result of overpopulation of states by dielectronic recombination.

– Prominences and filaments: imaging with high resolution these cool, filamentary structures suspended in the low corona and the precursors to spectacular eruptions into the interplanetary medium.

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Figure 5. ALMA whole-disk test data from February 2011. Observations were taken through a high level of attenuating atmospheric water. Normally special solar filters would have been inserted to attenuate strong solar radiation.

Structure and dynamics of the chromosphere

The emission of the quiet Sun as seen in the millimeter and submillimeter (Fig 5) arises mainly from thermal emission from the chromosphere, which is a thin layer above the temperature minimum region, where non-radiative heating first becomes manifest. The chromosphere constitutes a complex and rapidly evolving plasma structure. It is dynamically driven by the convection of the underlying photosphere. Its ionization structure changes dramatically from the temperature minimum to the base of the corona. ALMA’s potential for measuring lower solar atmosphere temperatures directly without complex modeling enables studies of energy flow in the region. Interpretation is difficult however as shocks propagate through the region, and complex magnetic field structure mediates flows. ALMA can provide subarcsecond snapshot images of the dynamic chromosphere to probe these phenomena.

Millimeter and submillimeter Solar Flares

Electrons are accelerated in solar flares to energies beyond 70 MeV and emit synchrotron emission and gamma-ray bremsstrahlung. BIMA observations ([Kundu et al. 2000, 2001] suggest that the synchrotron emission continues beyond the millimeter waves into the submillimeter regime. A subset of large solar flares present a “sub-THz” component which shows an inverted spectrum up to 405 GHz and could be even more prominent in the higher frequency bands accessed by ALMA. ALMA offers only a limited field of view but options to improve flare observations with ALMA are available. For instance, ALMA could be split into sub-arrays to cover a larger area, at the cost of lower sensitivity and spatial resolution. ALMA could also be operated not as an interferometer, but in a single-dish mode. Each antenna may be pointed at slightly different locations within the active region so that an entire active region is covered by a rapidly sampled mosaic.

Solar Recombination Lines

Clark et al. 2001 have reported the detection of the Hydrogen recombination line emission in the 350µm and 450&mum band—ALMA Bands 9 and 10. They resport a dramatic increase of the line-to-continuum ratio moving from the center of the solar disk to its limb. These lines may provide a new probe of the altitude structure of the quiet solar atmosphere above the limb and in prominences.

Prominences and Filaments

Filaments in the sun appear as cool snakelike structures suspended in the corona above magnetic neutral lines. Seen in absorption against the disk at long wavelengths, they appear in emission above the limb. They may undergo eruption in association with coronal mass ejection. Across the ALMA Bands, the emission changes from optically thick to optically thin, suggesting that ALMA observations with high spatial and temporal resolution across the wide ALMA frequency window may allow new insights into their physics.