Study of the circumstellar envelopesof evolved stars
Do Thi Hoai
Department of Astrophysics (DAP),Vietnam National Space Center (VNSC)

Ishigaki, November 2017

ContentIntroduction
Evolved stars: RS Cnc, EP Aqr, theRed Rectangle, Mira Ceti
Summary2

Evolution of the morphology
AGB stars

Intermediatestar

On the AGB, the CSE often evolves from a spherical shape tovery irregular morphologies. A common feature is the early
appearance of a bipolar outflow breaking spherical symmetry
into an axial one. It is commonly believed that binarity plays animportant role in this process. However, the details of themechanism governing the distribution of the angular momentum
and magnetic flux of the original star between the contracted
core and the CSE are not well understood.
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We study evolved stars using high resolution CO emission lines
observed by ALMA and IRAM telescopes (PdBI and 30-m single disk).
The stars feature a bipolar molecular outflow.
EP AqrW Aql
π1 GruRS CncπRed Rectangle

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RS Cnc
CO observations show clearly presence of two components,
with ~8(2) km/s winds and 4(0.8) 10-7 Msun/yr mass loss.
CO(2-1)
at 1.3mmCO(1-0)at 2.6mm

30m spectra, Libert et al. 2010

5Mapping across the source gives evidence for a strong bipolar flowcorresponding to the broad component.
Detailed inspection shows that this flow is along an axis making ~45o withthe plane of the sky (AI) and away from North (PA) by only ~10o.
Moreover, it is accompanied by a flow enhanced at the equator associatedwith the narrow component and normal to the axis of the bipolar flow.

25”

CO(1-0)(0.02 pc)
Libert et al. 2010,PdBI observation

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We developed a model with mass loss rate and wind radial
velocity decreasing smoothly from the poles to the equator,
adjusting the parameters by 2 minimization of the fit to thespectral maps.
RS CncCO(1-0)CO(2-1)7

Radial dependence of the CO(2-1) and CO(1-0) fluxes
A major difference between the CO(1-0) and CO(2-1) observations isthe broader extension of the former with respect to the latter.
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=1.1288

RS Cnc: a good test case for detailed studies
The model best fit for RS Cnc implies a significant velocity
gradient which may be interpreted in the frame of two extreme
scenarios: assuming either a constant mass loss rate or a constantwind velocity over some 105 years. We have shown that bothinterpretations are plausible, the former being only slightly
preferred.
We have also looked for a possible rotation of the envelope(ignored in the simple form of the model) and placed an upper
limit on the rotation velocity of ~1 kms–1 at a distance of 1 arcsec
from the star. These examples illustrate the need for observationsof high sensitivity and high spatial and spectral resolution.

EP Aqr
EP Aqr is at the beginning of its evolution on the AGB
The polar axis makes an angle of 13o to the line of sight

Poles T~ r−1.2

Equator T~
r−0.7

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Effective emissivity
Effective emissivity is defined as: ρeff (r, α, ω) = f(y, z, Vx)dVx/dx
Under the approximation of LTE and negligible absorption the effective
emissivity can be related simply to the density and temperature of the gas.
CO(1-0)CO(2-1)ρeffr2in meridian plane

The effective emissivity displays a significant asymmetry between the northand south hemispheres of the star.

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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 disk of gas.
Recently, ALMA observations in CO(3-2) & CO(6-5) having an order of magnitude
better resolution have become accessible.

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CO(6-5)/CO(3-2)

CO(3-2) EW

CO(6-5) EW

CO(6-5)/CO(3-2) map: evidence for a temperature distribution dominatedby the biconical structure down to low distances from the star.
The East-West asymmetry reveals a very clear rotation of the equatorial
region around the star axis.

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In meridian planeTemperatureDensity×r214

Gas kinematics
Gas velocities are modeled by using the values of the effective emissivities andfitting the CO(3-2) and CO(6-5) data together with a same velocity distribution.
Equator region: β0~0.8”, Vrot~1 km/s, k~1, Vrad~1.6 km/s
Polar regions: well described by parabolic meridian trajectories joining
smoothly between the torus and the star axis with a constant wind velocity of theorder of 6 to 7 km/s

Polar region

Equatorial region 15

Mira Ceti
Mira Ceti is one of the most studied binary stars.
Mira A, is an AGB star with a mass-loss rate of the order of 10−7 M☼/yr.
Mira B, is probably a white dwarf at a projected distance on the plane of thesky of ~0.5 arcsec from Mira A and at a position angle of 8◦ south of east.

CO(3-2) observed by ALMA cycle 2

Central region of theBlue-shifted arc in slow
radial expansion, ∼1.7 km/s circumbinary envelope

Red-shifted arcs

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Effective emissivity is reconstructed in space under the assumptionof a pure radial expansion at constant velocity of 7 km s−1

North-eastern outflow

South-western outflows

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The close environment of the Mira A+B pair
Close to the stars, we observe a mass of gas surrounding Mira B, with asize of a few tens of au, and having Doppler velocities with respect toMira B reaching ±1.5 km s−1, which we interpret as gas flowing from Mira
A towards Mira B.

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The analysis of CO data from EP Aqr, the Red
Rectangle and RS Cnc has made it possible for us todevelop a general methodology and to understand its
potential and limitations. While the quality of the datain terms of spectral resolution was usually sufficient, theneed for as good as possible spatial resolution and highsensitivity was overwhelming.
In this sense, ALMA opens a new era in terms of thereliability and precision being now at hand. It opens thedoor to analyses where the uncertainties on the data canbe evaluated reliably, allowing for χ2 values beingtranslated in terms of quantitative confidence levels fora given model.
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Thank you for your attention!

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