Nascent Wind Research Articles

On the nascent wind of oxygen-rich AGB stars: scrutiny of a sample of nearby stars

On the nascent wind of oxygen-rich AGB stars: scrutiny of a sample of nearby stars

Recent mass ejection from AGB star W Hya

We analyse archival ALMA (Atacama Large Millimeter/submillimeter Array) observations of two molecular line emissions, 12CO(3-2) and 29SiO(8-7), from oxygen-rich AGB (Asymptotic Giant Branch) star W Hya. Together with results of earlier VLT observations at visible and infrared wavelengths, our results suggest a two-component picture of the morpho-kinematics of the circumstellar envelope (CSE), one stable over time, at the scale of centuries, and the other variable, at the scale of years. The stable component consists of an approximately spherical shell of gas and dust expanding radially to a terminal velocity of ~5 km s−1at a distance of ~30 au from the star. It is found to display comparable features as seen in the CSE of R Dor, a star similar to W Hya. The variable component projects on the plane of the sky over a region confined to the neighbourhood of the star and elongated toward the north. Its very high density and sudden acceleration suggest an interpretation in terms of mass ejection initiated a few years ago. We discuss its properties in relation with earlier observations of dust formation in the same region. Our results offer a picture of the wind of W Hya that differs significantly from the picture that could be suggested by earlier analyses, giving evidence for a mass ejection that had been previously overlooked and underscoring new relations between the dust and gas emissions. They have an impact on the evidence published earlier for the presence of CO masers. They favour an interpretation in terms of convective cell ejections playing the main role in the generation of the nascent wind, the stable component of the CSE being seen as the result of many successive such events occurring in different directions at short time intervals.

Mira Ceti, Atypical Archetype

Abstract With the aim of unraveling the complexity of the morphokinematics of the circumstellar envelope (CSE) of Mira Ceti, we review, extend, and in some cases revisit Atacama Large Millimeter/submillimeter Array observations of the emission of the SiO(5–4) and CO(3–2) molecular lines. In addition, we present a detailed analysis of the optically thin 13CO(3–2) emission, which provides several important new results. In agreement with observations at infrared and visible wavelengths, we give evidence for the confinement and probably rotation of a dense gas volume within ∼50 au from the star and for a large SiO line width within ∼15 au. We show that the mass-loss process is episodic and takes the form of clumps having a very low SiO/CO abundance ratio compared with similar oxygen-rich long-period variables, probably a result of depletion on dust grains and photodissociation. We evaluate the mass-loss rate associated with the main clumps and compare it with values obtained from single-dish observations. We argue that the SiO emission observed in the southwestern quadrant is not related to the mechanism of generation of the nascent wind but to a mass ejection that occurred 11 years before the observations. We remark that Mira Ceti is not a good archetype in terms of its wind: models aiming at describing the very complex gas-dust chemistry in action in the CSE of oxygen-rich AGB stars may find it difficult to account for its peculiar features and small variations in the parameters deciding when and where mass loss can proceed significantly.

Morpho-kinematics of the wind of asymptotic giant branch star L2 Pup

ABSTRACT Single-dish observations of asymptotic giant branch star L2 Pup have revealed exceptionally low mass-loss rate and expansion velocity, challenging interpretations in terms of standard wind models. Recent VLT and ALMA observations have drawn a detailed picture of the circumstellar envelope within ∼20 au from the centre of the star: a nearly edge-on rotating disc of gas and dust, probably hosting a planetary companion near the star. However, these observations provide no direct information on the wind escaping the gravity of the star. This article uses ALMA observations of the 12, 13CO(3-2), 29SiO(8-7), 12CO(2-1), and 28SiO(5-4) line emissions to shed new light on this issue. It shows the apparent normality of L2 Pup in terms of the formation of the nascent wind, with important line broadening within 4 au from the centre of the star, but no evidence for a wind flowing along the disc axis. At larger distances, up to some 200 au from the centre of the star, the wind morpho-kinematics is dominated by a disc, or equatorial enhancement, expanding isotropically and radially with a velocity not exceeding some 5 km s−1, inclined in the north-west/south-east direction with respect to the plane of the sky. In addition, outflows of lower density are observed on both sides of the disc, covering large solid angles about the disc axis, contributing about half the flux of the disc. Such morphology is at strong variance with the expectation of a pair of back-to-back outflows collimated by the central gas-and-dust disc.

Prospects of wind energy deployment in Africa: Technical and economic analysis

This paper analyses the current status and barriers to wind energy deployment in Africa. We find that wind energy in Africa has rapidly grown by 740% from 0.74 GW (GW) in 2009 to 5.5 GW as of 2018. This has been spurred by continued falling costs, improved technologies, and political commitment. Notwithstanding the above, Africa still had installed only about 1% of its estimated wind capacity as of 2018, despite having competitive costs and speeds capable of deploying utility-scale wind farms. Wind LCOE costs in Africa have declined by 30% from an average of US cents 10 to 7 per kWh between 2010 and 2019. Furthermore, wind energy development on the continent is sporadic and skewed toward southern and northern Africa. While some countries have introduced targets for wind energy and instruments such as feed-in-tariffs, we find that the promotion, and scale-up of wind energy is lacking in many countries on the continent. Our findings add to the growing body of literature on the nascent wind industry in Africa. An equally important feature of this study is demystifying these challenges while making a case for shifting fossil fuel dominance in Africa by supporting clean energy transition.

Scaling up renewable energy in Africa: measuring wind energy through econometric approach.

Wind energy continues to make inroads in Africa due to falling costs and technological advancements. Most African countries are planning, exsiccating and connecting their renewable energy projects with national grid system with giving high propriety to energy security, sustainable energy consumption and low carbon emission. Many policies have been enacted by countries to promote the scaling up of wind energy and renewable energy in particular, across the globe. However, these policies have mixed effects on the deployment of wind energy. For this purpose, current study used panel data and fixed effects model for 17 African countries with wind installed generation capacity to determine the driver of wind energy development on the African continent between 2008 and 2017. The variables were grouped into three thematic areas: policy, socioeconomic, and country-specific factors. After conducting the analysis, socioeconomic variables (GDP, CO2, energy use) and energy security variables (energy import, electricity consumption) have significant effects in determining the scaling up of wind energy in Africa. However, the policy variables of FITs, licensing during, and Tax did not have significant effects on wind energy capacity addition for the case of Africa. This study adds to the drivers of nascent wind energy deployment literature in Africa. This study suggests that set of effecitive policies are deem necessary to scale up wind energy in Africa.

Observation of High Doppler Velocity Wings in the Nascent Wind of R Doradus

We study the morpho-kinematics in the nascent wind of AGB star R Doradus in the light of high Doppler velocity wings observed in the spectral lines of several species. We probe distances from the star between ∼10 and ∼100 au using ALMA observations of the emission of five different molecular lines. High Doppler velocity enhancements of the line emission are observed in the vicinity of the line of sight crossing the star, reminiscent of those recently interpreted as gas streams in the nascent wind of a similar AGB star, EP Aqr. They are present in both blue-shifted and red- shifted hemispheres but are not exactly back-to-back. They are accelerated at a typical rate of 0.7 km s\(^{−1}\) au\(^{−1}\) up to some 20 km s\(^{−1}\). Important differences are observed between the emissions of different molecules. We exclude an effect of improper continuum subtraction. However, in contrast to EP Aqr, the line of sight plays no particular role in the R Dor morpho-kinematics, shedding doubt on the validity of a gas stream interpretation. We discuss possible interpretations in terms of stellar pulsations or of rotation of the gas in the environment of the star. We conclude that, in the state of current knowledge, no fully convincing picture of the physics governing the production of such high velocities, typically twice as large as the terminal velocity, can be reliably drawn. New high resolution analyses of observations of the nascent wind of oxygen-rich AGB stars are needed to clarify the issue.

The nascent wind of AGB star R doradus: evidence for a recent episode of enhanced mass loss

ABSTRACT We analyse ALMA observations of the SO(JK = 65 − 54) emission of the circumstellar envelope of oxygen-rich AGB star R Dor, probing distances between 20 and 100 au from the star where the nascent wind is building up. We give evidence for a radial outflow covering very large solid angles and displaying strong inhomogeneity both in direction and radially: it contains multiple cores and has a radial dependence suggesting an episode of enhanced mass loss that occurred a century or so ago. Our analysis also confirms the presence of a previously observed rotating disc.

Observation of narrow polar jets in the nascent wind of oxygen-rich AGB star EP Aqr

Using ALMA observations of $^{12}$CO(2-1), $^{28}$SiO(5-4) and $^{32}$SO$_2$(16$_{6,10}$-17$_{5,13}$) emissions of the circumstellar envelope of AGB star EP Aqr, we describe the morpho-kinematics governing the nascent wind. Main results are: 1) Two narrow polar structures, referred to as jets, launched from less than 25 au away from the star, build up between $\sim$ 20 au and $\sim$ 100 au to a velocity of $\sim$ 20 \kms. They fade away at larger distances and are barely visible in CO data. 2) SO$_2$, SiO and CO emissions explore radial ranges reaching respectively $\sim$30 au, 250 au and 1000 au from the star, preventing the jets to be detected in SO$_2$ data. 3) Close to the star photosphere, rotation (undetected in SiO and CO data) and isotropic radial expansion combine with probable turbulence to produce a broad SO$_2$ line profile ($\sim$ 7.5 \kms\ FWHM). 4) A same axis serves as axis of rotation close to the star, as jet axis and as axi-symmetry axis at large distances. 5) A radial wind builds up at distances up to $\sim$ 300 au from the star, with larger velocity near polar than equatorial latitudes. 6) A sharp depletion of SiO and CO emissions, starting near the star, rapidly broadens to cover the whole blue-western quadrant, introducing important asymmetry in the CO and particularly SiO observations. 7) The $^{12}$C/$^{13}$C abundance ratio is measured as 9$\pm$2. 8) Plausible interpretations are discussed, in particular assuming the presence of a companion.