Evolutionary Impact of Stellar Mass Loss

Astro2020 Science White Paper

High Angular Resolution Astrophysics: Evolutionary Impact of Stellar Mass Loss

Thematic Areas: ☐ Planetary Systems ☐ Star and Planet Formation ☐Formation and Evolution of Compact Objects ☐ Cosmology and Fundamental Physics Stars and Stellar Evolution ☐Resolved Stellar Populations and their Environments ☐Galaxy Evolution ☐Multi-Messenger Astronomy and Astrophysics

Principal Author: Name: Douglas Gies Institution: Georgia State University Email: [email protected] Phone: 404 413 6021

Co-authors: (names and institutions) Stanley P. Owocki, University of Delaware Stephen Ridgway, National Optical Astronomy Observatory Theo ten Brummelaar, GSU CHARA Array

Abstract: Supernovae and stellar winds are the ways that the nuclear processed gas of stars is returned to the interstellar medium to form new generations of metal enhanced stars and planets. Advances in high resolution techniques are giving us the first glimpse of how mass loss processes are acting in stars of all kinds. They will help document stellar wind asymmetries in massive stars, probe the physics of eruptive mass loss in evolved stars, and record the details of disk formation and dynamics of the gas disks of Be stars. Interferometric observations are particularly powerful for the investigation of the molecular outflows and dust creation in the winds of Asymptotic Giant Branch Stars, revealing the details of how pulsational and convective motions can launch mass loss and eventually disperse the envelope and create a planetary nebula. The presence of a binary companion can create wind-wind collisions that sculpt large equatorial flows, and interactions in close binary systems can lead to systemic mass loss and the creation of circumbinary disks. Here we discuss how high angular resolution investigations will explore the processes of mass ejection at unprecedented photospheric scales.

Introduction Stellar mass loss is one of the primary means of chemical enrichment and evolution in the universe. Stellar outflows eject nuclear processed gas and create dust particles that set the stage for the current epoch of star and planet formation. Stellar mass loss processes are important throughout a star’s lifetime: limiting mass accretion during birth, controlling spin- down on the main sequence, determining horizonal branch morphology, transforming an AGB star into a white dwarf progenitor, and shaping the path toward supernovae in massive stars. High angular resolution methods help us probe the geometry and physical conditions of the outflows from close to far from the star. Here we present a series of questions about mass loss processes that can be addressed in the next decade through advances in high angular resolution methods and instrumentation.

Massive Star Winds The extreme radiation fields of the massive stars provide the momentum that drives stellar wind mass loss in Wolf-Rayet stars, blue supergiants, B[e] supergiants, and Luminous Blue Variable stars (LBVs). These line-driven winds are coupled to the stellar luminosity and metallicity (Vink 2018), and the mass loss rates are sufficient to reduce the stellar mass by a significant fraction over the H-burning lifetime. However, massive stars are also commonly members of close binary systems, and binary-related mass loss/transfer must play a significant role in determining their final, pre-supernova masses (Smith 2014). New observations are needed to understand the range of mass loss processes that massive stars experience.

Are stellar winds structured by rotation and instabilities? Massive stars are often rapid rotators with rotational frequencies approaching W=1, where centrifugal and gravitational accelerations balance at the equator. Rotation will cause polar flattening and heating, and models predict greater polar mass loss (Fig. 1). Observational tests are critical, because polar winds carry away little angular momentum so that the star may be a very fast rotator at the time of the supernova, yielding the progenitors of �-ray bursters (Cantiello et al. 2007). High angular resolution images of the wind of the LBV P Cygni show a spherical wind (Fig. 1) while that of � Car is elongated (Fig. 2). Models indicate that the winds are subject to large-scale instabilities (Co-rotating Interacting Regions; Cranmer & Owocki 1996) and small-scale clumping (Sundqvist et al. 2018). Thus, real winds may have complex forms as evidenced in the complicated nebulae that surround massive stars (Fig. 1; see Toalá et al. 2015).

FIG. 1 – Left: Isodensity contours for winds of rotating stars (Dwarkadas & Owocki 2002). Middle: P Cygni (Richardson et al. 2013). Right: Nebula around WR 136 (Moore et al. 2000).

How does eruptive mass loss occur prior to a supernova? Evolved, luminous stars are probably subject to episodes of eruptive mass loss that create massive outflows (Owocki et al. 2019). These are observed in the historic outbursts recorded for � Carinae and other LBVs (Fig. 2). The resulting circumstellar nebulae are sometimes energized by a supernova explosion, creating ring-like structures such as observed in SN1987A (Fig. 2). A close example of a SN progenitor is the red supergiant Betelgeuse, and high angular resolution observations have probed the complex structures in its surroundings (Fig. 2). These observations document the dust forming regions at tens of stellar radii, while interferometry with higher spectral resolution has probed the closer molecular layer (Montarges et al. 2014). Spectro-interferometric imaging is now providing the means to measure directly the kinematics of mass loss processes in red supergiants like Antares (Ohnaka et al. 2017).

FIG. 2 – � Carinae (NASA, ESA); SN1987A (NASA, ESA); Betelgeuse (Kervella et al. 2011).

How do circumstellar disks form and evolve around Be stars? The fastest rotating massive stars eject gas disks due to the action of rotation and pulsational beating (Baade et al. 2018). The disks of such B-emission line or Be stars are intrinsically variable on timescales of years (Dalla Vedova et al. 2017) as the disk gas is lost or returned to the star. The disks are resolved through interferometry and display Keplerian motion (Fig. 3). The disks are dynamic and exhibit one or two-armed spirals (� Tau; Fig. 3), disk precession (� Tau; Schaefer et al. 2010), and gaps and tidal arms formed by low mass companions (HR 2142; Fig. 3). High resolution observations of Be star disks offer the means to investigate in relatively nearby targets those disk processes found throughout astrophysics.

FIG. 3 – Models of the disk of g Cas (Gies et al. 2007); the one-armed spiral density wave of z Tau (Carciofi et al. 2009); a gap created by the companion of HR 2142 (Peters et al. 2016).

3 Mass Loss in AGB Stars Mass loss accelerates as stars ascend the Asymptotic Giant Branch (AGB) eventually removing the outer layers that include mixed and nuclear enriched gas. The extent of mass loss will impact the amount returned to the ISM and the masses of white dwarf remnants. High angular resolution studies are key to understanding how mass loss occurs. According to Höfner & Olofsson (2018), “High angular resolution observations indicate at what distances from the stars dust condensation occurs, and they give information on the chemical composition and sizes of dust grains in the close vicinity of cool giants. These are essential constraints for building realistic models of wind acceleration and developing a predictive theory of mass loss for AGB stars, which is a crucial ingredient of stellar and galactic chemical evolution models.”

What kinds of environment promote molecular and dust formation? There is much observational evidence of molecular formation in a MOLsphere beyond the photosphere (Tsuji 2000). The denser regions of the outflow are favorable for dust formation with a composition that varies from amorphous carbon to polycyclic aromatic hydrocarbons (PAHs) as the star evolves to a planetary nebula (Sloan 2017). Mid-infrared and radio interferometry help determine at what radii molecular and dust formation occurs (Zhao-Geisler et al. 2012; Karovicova et al. 2013; Perrin et al. 2015). Pulsation is found with enhanced mass loss (Paladini et al. 2017), but mass loss also exists among non-pulsators (Ohnaka et al. 2019).

How do photospheric motions influence AGB mass loss? Many AGB stars are Long Period Variables and Mira pulsators, and the combined motions of oscillations and giant convection cells create shocks that eject gas. Dust formation can occur in dense shock regions, and once formed, the dust opacity helps launch the wind acceleration (Höfner & Olofsson 2018). Models of pulsational and convection driven mass loss (Fig. 4) show how these forces push molecular gas into a complex extended atmosphere, and interferometric observations confirm the presence of structures (Wittkowski et al. 2017). VLTI interferometry of dust shells around Carbon Stars show that they are more extended among pulsators than non-pulsators (Rau et al. 2017). Time series observations over a pulsation cycle are beginning to show how the molecular layers respond to the pulsational piston (Wittkowski et al. 2018) and how the dust size and production rate vary with pulsational phase (Ohnaka et al. 2017). These processes help explain the clumpy appearance of dust around W Hya (Fig. 4) and other stars.

FIG. 4 – 3D model of intensity in a water vapor band (Wittkowski et al. 2016); clumpy dust emission around W Hya (Ohnaka et al. 2017); ALMA 12CO map of OH30.1-0.7 (Decin et al. 2019).

4 What processes dominate at the late stages of AGB evolution? As AGB stars expand with time, their pulsation periods increase and their mass loss rates surge. -5 -1 The highest mass loss rates reach > 10 M⊙ yr in extreme OH/IR stars during a “superwind” stage that may last 104 yr, long enough to remove the H-envelope and create a planetary nebula (Höfner & Olofsson 2018). However, high resolution observations show that some of these extreme cases are actually dense disks formed through the influence of a binary companion (see the case of OH30.1-0.7 in Fig. 4; Decin et al. 2019). Thus, the superwind cases may be more prolonged or repetitive in nature than previously thought. Mass loss may continue after the planetary nebula stage when the remnant re-expands as an R CrB object and ejects carbon dust plumes that may occult the star (Clayton et al. 2011; Chesneau et al. 2014).

Mass Loss from Binary Systems Binary stars are commonplace and the proximity of a binary companion can have an enormous influence on the mass loss properties of both components. A wide binary may imprint structure on the wind of the more luminous star, and a close binary may experience Roche lobe overflow which may spill over into the circumbinary environment. High angular resolution observations provide the means to explore binary interactions directly.

In what situations do wind-wind collisions occur? If both stars have winds, then the winds will collide to form a bow shock at the interface. Speckle masking observations by Tuthill et al. (2008) demonstrated the existence of a dusty pinwheel nebula in the binary system WR104 (Fig. 5). Here the powerful wind of the Wolf-Rayet primary sweeps around the weaker wind of an OB-companion, and dust is formed in the compressed gas of the shock region. This dust moves slowly outward in a spiral trailing the binary motion. One of the most dramatic wind-wind collisions occurs between the massive components of � Carinae in the equatorial skirt of its nebula (Fig. 2). This is an eccentric orbit binary that brings the components very close every five years, and during the short duration periastron passage the high densities and temperatures of the collision zone create a spike in X- rays (Fig. 6). The high angular resolution of the VLTI and Gravity combiner have led to maps of the gas asymmetries formed by the arms of the bow shock (Gravity Collaboration 2018).

FIG. 5 – The dust pinwheel formed by wind-wind collision in WR104 (Tuthill et al. 2008); model RLOF and circumbinary disk (MacLeod et al. 2018); double-torus of RY Scuti (Smith et al. 2011).

FIG. 6 – Model of the wind-wind collision in � Carinae (Madura et al. 2013). The wide bow shock region found at apastron (left) collapses as the stars close in at periastron (center) and re- sculpts the interaction region as the stars separate (right).

How is mass lost in interacting binaries? Close binary stars are destined to exchange mass by Roche lobe overflow (RLOF) in which the initially more massive donor transfers mass and angular momentum to the gainer star. The gainer may be spun up in the process to near critical rotation, so that additional accretion is severely limited. In such cases, gas may leak out along the axis joining the stars near the positions of the outer Lagrangian points and create a circumbinary disk (see model in the middle panel of Fig. 5). High angular resolution observations are beginning to explore the properties of circumbinary disks. HST observations by Smith et al. (2011; see left panel of Fig. 5) show the double gas torus surrounding the massive binary RY Scuti (unresolved) that exists within a larger dust torus. The CHARA Array interferometer imaged the gas disk that occults the F-supergiant � Aurigae (Kloppenborg et al. 2010), and this disk was probably formed through the interaction of two B-stars (Lissauer & Backman 1984). Lower mass binaries that experience the AGB stage may also lose mass into a wide circumbinary disk, and VLTI/MIDI interferometry observations in the infrared N-band have led to the detection of the inner hot regions of circumbinary disks in post-AGB systems (Hillen et al. 2017). High resolution observations probe the details of outflows, shed light on non-conservative binary evolution, and reveal those critical interaction processes that transform the stars.

Promise of High Angular Resolution Observations New capabilities in interferometry will allow us to explore mass loss processes on scales smaller than the stellar radius, so that we can investigate how mass ejection occurs near the photosphere. In particular, advances in spectro-interferometry and temporal studies will show how the chemical, kinematical, and physical properties of outflows vary as a function of distance from the star. These will lead to a new understanding of the processes that drive stellar evolution and the chemical enrichment of galaxies. Such advances will require funding support to (1) maintain and expand high angular resolution facilities, (2) invest in the training of the next generation of scientists, and (3) provide the means for broad community access to the best instruments for high angular resolution observations.

References

Baade, D., Pigulski, A., Rivinius, Th., et al. 2018, A&A, 610, A70 Cantiello, M., Yoon, S.-C., Langer, N., & Livio, M. 2007, A&A, 465, L29 Carciofi, A. C., Okazaki, A. T., le Bouquin, J.-B., et al. 2009, A&A, 915 Chesneau, O., Millour, F., De Marco, O., et al. 2014, A&A, 569, L4 Clayton, G. C., Sugerman, B. E. K., Stanford, S. A., et al. 2011, ApJ, 743, 44 Cranmer, S. R., & Owocki, S. P. 1996, ApJ, 462, 469 Dalla Vedova, G., Millour, F., Domiciano de Souza, A., et al. 2017, A&A, 601, A118 Decin, L., Homan, W., Danilovich, T., et al. 2019, Nature Astron., in press (arXiv:1902.09259) Dwarkadas, V. V., & Owocki, S. P. 2002, ApJ, 581, 1337 Gies, D. R., Bagnuolo, W. G., Jr., Baines, E. K., et al. 2007, ApJ, 654, 527 Gravity Collaboration, Sanchez-Bermudez, J., Weigelt, G., et al. 2018, A&A, 618, A125 Hillen, M., Van Winckel, H., Menu, J., et al. 2017, A&A, 599, A41 Höfner, S., & Olofsson, H. 2018, A&ARv, 26, 1 Karovicova, I., Wittkowski, M., Ohnaka, K., et al. 2013, A&A, 560, A75 Kervella, P., Perrin, G., Chiavassa, A., et al. 2011, A&A, 531, A117 Kloppenborg, B., Stencel, R., Monnier, J. D., et al. 2010, Nature, 464, 870 Lissauer, J. J., & Backman, D. E. 1984, ApJ, 286, L39 MacLeod, M., Ostriker, E. C., & Stone, J. M. 2018, ApJ, 863, 5 Madura, T. I., Gull, T. R., Okazaki, A. T., et al. 2013, MNRAS, 436, 3820 Montarges, M., Kervella P., Perrin G., et al. 2014, A&A, 572, A17 Moore, B. D., Hester, J. J., & Scowen, P. A. 2000, AJ, 119, 2991 Ohnaka, K., Hadjara, M., & Maluenda Berna, M. Y. L. 2019, A&A, 621, A6 Ohnaka, K., Weigelt, G., & Hofmann, K.-H. 2017, Nature, 548, 310 Ohnaka, K., Weigelt, G., & Hofmann, K.-H. 2017, A&A, 597, A20 Owocki, S. P., Hirai, R., Podsiadlowski, P., & Schneider, F. R. N. 2019, MNRAS, 485, 988 Paladini, C., Klotz, D., Sacuto, S., et al. 2017, A&A, 600, A136 Perrin, G., Cotton, W. D., Millan-Gabet, R., & Mennesson, B. 2015, A&A, 576, A70 Peters, G. J., Wang, L., Gies, D. R., & Grundstrom, E. D. 2016, ApJ, 828, 47 Rau, G., Hron, J., Paladini, C., et al. 2017, A&A, 600, A92 Richardson, N. D., Schaefer, G. H., Gies, D. R., et al. 2013, ApJ, 769, 118 Schaefer, G. H., Gies, D. R., Monnier, J. D., et al. 2010, AJ, 140, 1838 Sloan, G. C. 2017, Planetary & Space Sci, 149, 32 Smith, N. 2014, ARA&A, 52, 487 Smith, N., Gehrz, R. D., Campbell, R., et al. 2011, MNRAS, 418, 1959 Sundqvist, J. O., Owocki, S. P., & Puls, J. 2018, A&A, 611, A17 Toalá, J. A., Guerrero, M. A., Ramos-Larios, G., & Guzmán, V. 2015, A&A, 578, A66 Tsuji, T. 2000, ApJ, 540, L99 Tuthill, P. G., Monnier, J. D., Lawrance, N., et al. 2008, ApJ, 675, 698 Vink, J. 2018, A&A, 619, A54 Wittkowski, M., Chiavassa, A., Freytag, B., et al. 2016, A&A, 587, A12 Wittkowski, M., Hofmann, K.-H., Höfner, S., et al. 2017, A&A, 601, A3 Wittkowski, M., Rau, G., Chiavassa, A., et al. 2018, A&A, 613, L7 Zhao-Geisler, R., Quirrenbach, A., Köhler, R., & Lopez, B. 2012, A&A, 545, A56