Study of the gas envelopes of evolved stars and protostars
P.T. Nhung on behalf of the Department of AstroPhysics (DAP) Vietnam National Satellite Center (VNSC)
Hanoi, December 2016 1 Content Introduction Evolved stars: RS Cnc, the Red Rectangle, Mira Ceti Protostars: L1527, GG Tau
2 2 Star formation Introduction
For now five years or so, we work exclusively in radio astronomy, using data from major international observatories on stellar physics (birth and death of stars) and high redshift galaxies (the early Universe).
Evolved stars
3 Evolved stars
Sun-size stars that have burned enough of their hydrogen into helium grow a core that becomes hot enough for its electrons form a Fermi gas, leaving the helium nuclei aggregate into carbon and oxygen.
The stellar envelope around the core blows up to gigantic sizes (~ hundreds AU), cooling down to temperatures in the ten to few hundred K range, where molecules emit at millimetre/ sub–millimetre wavelengths. This is the emission that we study, in particular from rotating CO molecules. 4 Star formation
Protostars are to some extent the time reversed process: molecular clouds, which happen to be in an interstellar region where important gravity fluctuations are present, start collapsing, gas falling-in toward the cloud centre.
Conservation of angular momentum causes the rotation velocity to increase in the process and a disc forms; its temperature and density become so high that atoms ionize and hydrogen nuclei start fusing into helium to form a new star.
5 Similarities between the two processes are many: - Dust plays a very important role: Evolved stars: as the main motor of expansion Protostars: dust accumulates in the disc plane, some dust grains aggregate to form larger and larger bodies, ultimately planetesimals and planets. - the importance of symmetries: unanswered questions related to the role of magnetic fields and binaries, an obvious source of symmetry breaking. - the importance of periodic oscillations, taking the form of pulsed accretion in protostars and of thermal pulses in AGB stars.
6 We normally use observations made with radio interferometers: Plateau de Bure (6 antennas), VLA (27) and ALMA (66). We analyze observations partly on our own and partly in collaboration with French astronomers. The evolved stars often feature a bipolar molecular outflow, the protostars the formation of a disc.
EP Aqr GG Tau W Aql Π1 Gru RS Cnc
Red Rectangle L 1527 49 Ceti Disc Jets 7 7 EVOLVED STARS RS Cnc, the Red Rectangle, Mira Ceti
8 RS Cnc CO observations show clearly presence of two components, -7 with ~8(2) km/s winds and 4(0.8) 10 Msun/yr mass loss.
CO(2-1) at 1.3mm
CO(1-0) at 2.6mm
30m spectra, Libert et al. 20109 Mapping across the source gives evidence for a strong bipolar flow corresponding to the broad component. Detailed inspection shows that this flow is along an axis making ~45o with the plane of the sky (AI). It is accompanied by a flow enhanced at the equator associated with the narrow component.
25” CO(1-0)
(0.02 pc) Libert et al. 2010, PdBI observation
10 We developed a model with mass loss rate and wind radial velocity decreasing smoothly from the poles to the equator, adjusting the parameters by Χ2 minimization of the fit to the spectral maps. RS Cnc (PdBI) CO(1-0) CO(2-1)
11 11 Hoai et al. 2014 Radial dependence of the CO(2-1) and CO(1-0) fluxes
The radial dependence of the ratio of the CO(2-1) to CO(1-0) emission constrains the values taken by T(r)=Ar─α. A detailed comparison reveals the need for the gas to reach lower temperatures than expected over the radial range probed here.
α=0.7 α=1.12
Nhung et al. 2014 12 12 The Red Rectangle The Red Rectangle is a Post-AGB source, distance d=710 pc Its axis is perpendicular to the line of sight
In 2005, PdBI observations in CO(1-0) & CO(2-1) having a resolution ~1”: suggested the presence of a disc of gas. Recently, ALMA observations in CO(3-2) & CO(6-5) having an order of magnitude better resolution have become13 accessible. (3-2)EW (6-5)EW (6-5)/(3-2)
The East-West Doppler velocity asymmetry reveals a very clear rotation of the equatorial region about the star axis.
CO(6-5)/CO(3-2) intensity map: evidence for a temperature distribution dominated by the biconical structure down to low distances from the star.
14 14 Temperature Density×r2
Assuming rotation symmetry about the star axis we reconstruct the gas morphology in space. 15 15 Gas kinematics Polar regions: parabolic meridian trajectories joining smoothly between the equatorial torus and the star axis with a constant wind velocity of the order of 6 to 7 km/s. Equator region: spiralling trajectories with ~1 km/s rotation and ~1.6 km/s expansion.
16 Polar region Equatorial region16 Mira Ceti, a famous binary
−7 Mira A, is an AGB star with a mass-loss rate of the order of 10 M☼/yr. Mira B, is probably a white dwarf at a projected distance on the plane of the sky of ~0.5 arcsec from Mira A. CO(3-2) observed by ALMA cycle 2
Blue-shifted arc in slow Central region of the Red-shifted arcs radial expansion, ∼1.7 km/s circumbinary envelope 17 The very complex morphology reveals outflows in the North-east/ South-west direction. We reconstruct the effective emissivity in space under the assumption of a pure radial expansion at constant velocity of 7 km s−1. It gives evidence for detached arcs and cavities suggesting violent past events. North-eastern outflow South-western outflows
18 18 The close environment of the Mira A+B pair Close to the stars, we observe a mass of gas surrounding Mira B, with a size of a few tens of au, and having Doppler velocities with respect to Mira B reaching ±1.5 km s−1 -> gas flowing from Mira A towards Mira B.
19 The analysis of CO data from the Red Rectangle, RS Cnc, EP Aqr... has made it possible for us to develop a general methodology and to understand its potential and limitations.
While the quality of the data in terms of spectral resolution was usually sufficient, the need for good spatial resolution and high sensitivity was overwhelming.
In this sense, ALMA opens the door to analyses where the uncertainties on the data can be evaluated reliably, allowing for χ2 values being translated in terms of quantitative confidence levels for a given model.
2020 PROTOSTARS L1527, GG Tau
21 L1527, a young protostar
C18O(2-1) - ALMA L1527, at a distance of 140 pc, has a mass of 0.2 solar masses and is surrounded by a flared rotating envelope of about one solar mass.
The disc is seen edge-on.
Emissivity Doppler velocity
2222 We reconstruct the gas morphology and –nrot kinematics assuming rotation invariance Vrot=V0r about the disc axis. The rotation velocity approaches Keplerian at small distances. In-fall is suppressed on the disc axis (hot outflow) and in the disc plane (gas freezing on dust grains). Kinematics provides a measurement of the star mass. 23 GG Tau, a triple protostar The separation between the main protostar and the close-binary is 35 au. The former consists of gas and dust in a rotating torus making an angle of 35o with the plane of the sky. Tang et al. (2016) have published observations of 13CO(3-2) ALMA data. Together with them we have extended the analysis to obtain further detail.
The kinematics is dominated by circular trajectories with Keplerian rotation velocity well described by a form ~2.21 (r/2 arcsec)–0.48 km/s. We have placed an upper limit of 8% of the rotation velocity on a possible in-fall velocity. 2424 The future
What we need most are observations of high quality to the analysis of which we can contribute. We shall continue our work on evolved stars (ALMA and PdBI proposals in collaboration with Paris), protostars (ALMA proposal in collaboration with Bordeaux) and remote galaxies (ALMA proposal). We will continue to exploit the ALMA public data.
25 25 We are making extensive use of the open data policy of the ALMA collaboration, who make their observations publicly available one year after collection. The data are reduced and a help desk provides support to handle them properly. This generous policy is an invaluable asset to teams such as ours, working in developing countries having otherwise no direct access to frontier astrophysics. We are immensely indebted and grateful to the ALMA partnership.
We are working toward collaborating with Asian countries, in particular Japan, South Korea, Taiwan and China.
Thank you! 26 26