RESEARCH NEWS & VIEWS creating a horseshoe-shaped structure — the wake — that extends as the animal moves forward (Fig. 1a). Owing to the laws of conservation, the circulation around the wings is equal in strength to the circulation around each of the two tip vortices. Air flows down through the middle of the wake, and the steady change in momentum as this downwash region elongates is equal to the lift created by the wings. Any animal flying behind another should avoid the downwash region, which would literally push them earthward. Just outside the downwash zone, however, there is a small region of upwash, created by the circular flow of air in the tip vortices. By careful placement of its own wing tip, a trailing bird can exploit the upwash generated by the tip vortex of a leading bird, thereby generating lift more efficiently and reducing its flight cost (Fig. 1b). For an aeroplane pilot, keeping one wing precisely within the small upwash region of a leading plane’s tip vortex is tricky enough, but for a bird the problem is further complicated by the flapping wings of its neighbours, which create tip vortices that undulate up and down. A bird that is following another bird must carefully adjust its own flapping motion, not in perfect temporal synchrony with the leader, but rather at a precise phase lag that tracks the tip vortex as it oscillates. When flying most efficiently, all the birds in a formation should flap with a precise metachrony (a wave-like synchrony), such that the flapping phase changes systematically from the leader to each bird down the line. Several theoretical studies3,5–8 have predicted how birds flying in formation could optimize energy savings by tuning their spacing and wing motion, and geese flying in a V formation have been observed to align their body positions in a way that might save energy7. But until now, no experimental data have shown that birds are capable of the precise adjustment of flapping phase that is necessary to track undulating tip vortices. Northern bald ibises (Geronticus eremita) often fly in a V formation when they migrate. To examine their behaviour during formation flight, Portugal and co-workers mounted custom-built data loggers on 14 ibises that accurately measured the body position and flapping dynamics of each bird. The authors found that trailing birds flew so as to keep their inner wing in the upwash zone of the bird in front of them, just as predicted by theory3. This requires not only correct regulation of body position, but also proper adjustment of the flapping phase, so that each bird’s wing tip follows the undulating tip vortex of the individual in front of it. Because the birds occasionally shifted position within the formation, situations occurred in which trailing birds briefly flew directly behind a leading bird. In these situations, the trailing birds tended to flap their wings in strict antiphase with the leading bird, thereby minimizing the negative interactions with the downwash region of the wake. This change in behaviour indicates
that the ibises actively adjust their flapping pattern under different conditions. Although these findings are qualitatively consistent with theoretical predictions, many challenging questions remain. For example, how much energy do the birds actually save? The best existing evidence that V formations save a significant amount of energy is that pelicans have a lower heart rate and show reduced flapping frequency when flying in formation compared with when flying solo9. Accurate measurements of metabolic rate will be crucial for a more precise understanding of the underlying aerodynamics of formation flight and for greater insight into the ecology of bird migration. Do ibises and other birds instinctively flap in an efficient manner when flying in formation, or do they learn to adjust their body position and wing motion because it ‘feels’ easier? And if the strategy is so useful, why do many species of small migrating birds not fly in a V formation? Might the benefits of formation flying decrease with body size, or is the requisite control of body position and wing motion more difficult for smaller,
faster-flapping birds? Although our understanding of V formations has improved, there is still much to ponder when looking skyward on late summer days. ■ Florian T. Muijres and Michael H. Dickinson are in the Department of Biology, University of Washington, Seattle, Washington 98195-1800, USA. e-mail: [email protected] 1. Ning, S. A. Aircraft Drag Reduction Through Extended Formation Flight. PhD thesis, Stanford Univ. (2011). 2. Vachon, M. J., Ray, R., Walsh, K. & Ennix, K. in AIAA Atmos. Flight Mech. Conf. Exhib. Abstr. 2002-4491 (Am. Inst. Aeron. Astronautics, 2002); http://dx.doi. org/10.2514/6.2002-4491 3. Willis, D. J., Peraire, J. & Breuer, K. S. 25th AIAA Appl. Aerodynam. Conf. Abstr. 2007-4182 (2007); http://dx.doi.org/10.2514/6.2007-4182 4. Portugal, S. J. et al. Nature 505, 399–402 (2014). 5. Lissaman, P. B. S. & Lundry, J. L. J. Aircr. 5, 17–21 (1968). 6. Hummel, D. J. Theor. Biol. 104, 321–347 (1983). 7. Badgerow, J. & Hainsworth, F. J. Theor. Biol. 93, 41–52 (1981). 8. Maeng, J.-S., Park, J.-H., Jang, S.-M. & Han, S.-Y. J. Theor. Biol. 320, 76–85 (2013). 9. Weimerskirch, H., Martin, J., Clerquin, Y., Alexandre, P. & Jiraskova, S. Nature 413, 697–698 (2001).
ASTR O PH YSI CS
Black hole found orbiting a fast rotator Stars of spectral type ‘Be’ are often found with neutron stars or other evolved analogues, but a black-hole companion has never been spotted before. Optical emission from a black hole’s surroundings has given it away. See Letter p.378 M . V I R G I N I A M C S WA I N
S
tellar-mass black holes, which are formed from the gravitational collapse of massive stars, are unusually scarce in our Universe. It is not yet clear whether there are fewer of them than expected or whether they are just hard to find, but either way they are deserving of their exotic reputation. Therefore, the discovery by Casares et al.1, reported on page 378 of this issue, of a stellar-mass black hole orbiting around a star dubbed MWC 656 is like finding a needle in a haystack. This black hole does not emit X-ray radiation — as black holes are expected to do — so it could be the first sign of a large population of ‘quiescent’ black holes. MWC 656 is itself interesting because it is surrounded by a dense outflow from the star’s equator caused by a combination of its fast rotation (the projected rotational velo city2 is about 300 kilometres per second) and pulsations that can eject material from the equator3. The resulting circumstellar disk produces spectral emission lines from hydrogen and other elements, meriting its classification
2 9 6 | N AT U R E | VO L 5 0 5 | 1 6 JA N UA RY 2 0 1 4
© 2014 Macmillan Publishers Limited. All rights reserved
as a ‘B-emission’ or ‘Be’ star. Fast rotation is a requirement in the formation of Be stars, and they probably acquire their high angular momentum during mass transfer from a massive companion star that eventually explodes as a supernova. In fact, many Be stars are found with highly evolved companions, usually neutron stars that are remnants of the postsupernova massive stars4. A few Be stars have companion stars that have been stripped down to just their helium cores5, but such a hot object is ruled out in the case of MWC 656 because there is no significant ultraviolet-light contribution coming from anywhere other than the Be star. MWC 656 is the first Be star to have a black-hole companion detected (Fig. 1). Casares et al. have identified emission lines from helium plasma that is trapped in an accretion disk around the black hole, as well as emission from the disk around the Be star, that provide a robust measurement of the mass ratio between the star and the black hole. Such emission lines are notoriously difficult to measure: they are broad and often asymmetric, complicating the usual procedures used to
NEWS & VIEWS RESEARCH
Circumstellar disk
Be star
50 Years Ago
Black hole
Accretion disk
Figure 1 | A Be star with a black-hole companion. Casares et al.1 have detected a quiescent black hole circling a Be star by measuring optical emission from the black hole’s accretion disk and from the large disk around the star. As the black hole orbits the star, some material from the circumstellar disk is transferred to the accretion disk. However, the high angular momentum of material in the accretion disk inhibits it from falling into the black hole, so there will be a gap between the accretion disk and the black hole, and the black hole remains quiescent.
measure the line centres, but the authors have taken care to reduce any underlying systematic errors. Taken together with the mass of the Be star, which is about 10–16 solar masses, the measured ratio implies that the black hole has a mass of between 3.8 and 6.9 solar masses. In studies of stellar evolution, conventional wisdom tells us that stellar-mass black holes form during the collapse of the cores of very massive stars — those with masses more than 25 times that of the Sun6 — once the stars exhaust their fuel, and that the collapse is possibly accompanied by a supernova. The supernovae that massive stars (8–25 solar masses) undergo are expected to produce neutron star remnants instead. These massive stars tend to form within close groups of stars, so binary star systems are the norm, and triple and quadruple systems are not unusual. The catastrophe of a supernova in a binary has dramatic consequences: if more than half of the total-system mass is lost, or if ‘kick’ velocity from the explosion propels the newly formed supernova remnant with enough momentum, the remnant and companion star could fly off in opposite directions7. But if the companion star does remain gravitationally bound to the remnant, an X-ray binary is formed: the black hole or neutron star remnant interacts with the remaining star to produce X-ray emission. Theorists predict that stellar-mass black holes are abundant. If this is so, we should find them all over the Milky Way. Many of them ought to be bound in X-ray binaries, whereas others should be freely floating through space. There are probably tens of millions of massive stars in the Milky Way that could potentially collapse into black holes, but there are only about 50 stellar-mass black holes known with good confidence8. X-ray studies of young star-forming regions such as the Carina Nebula, which might contain at least a few recent supernovae products, have not found any black holes9. Even large sky surveys are coming up with little as they search for the subtle brightness variations
of stars whose light bends around a foreground black hole that passes in front of the star (an effect known as microlensing)10. MWC 656 presents a rare opportunity to study mass transfer, angular momentum and accretion-disk physics around a quiescent black hole. Casares et al. find a hint of a hotspot on the black hole’s accretion disk that suggests that mass is pulled away from the Be star’s disk, crashing into the accretion disk when the stars make their closest approach during their orbit around one another. The absence of X-ray emission from this system is evidence that material is not channelled into the black hole; rather, it must be retained in a holding pattern within the accretion disk. Gas in the outer regions of the Be star’s disk will have high angular momentum, which will be transferred to the accretion disk during the mass transfer. Without an efficient mechanism to remove this angular momentum, accretion will be suppressed and the black hole will remain quiet. If there exists a larger population of Be star– black-hole binaries, such quiescence is probably the rule, not the exception. Casares et al. have shown us a way to find them. ■ M. Virginia McSwain is in the Department of Physics, Lehigh University, Bethlehem, Pennsylvania 18015, USA. e-mail: [email protected] 1. Casares, J. et al. Nature 505, 378–381 (2014). 2. Williams, S. J. et al. Astrophys. J. 723, L93–L97 (2010). 3. McSwain, M. V., Huang, W., Gies, D. R., Grundstrom, E. D. & Townsend, R. H. D. Astrophys. J. 672, 590–603 (2008). 4. Liu, Q. Z., van Paradijs, J. & van den Heuvel, E. P. J. Astron. Astrophys. 455, 1165–1168 (2006). 5. Gies, D. R. et al. Astrophys. J. 493, 440–450 (1998). 6. Fryer, C. L. Astrophys. J. 522, 413–418 (1999). 7. Brandt, N. & Podsiadlowski, P. Mon. Not. R. Astron. Soc. 274, 461–484 (1995). 8. Belczynski, K., Wiktorowicz, G., Fryer, C. L., Holz, D. E. & Kalogera, V. Astrophys. J. 757, 91 (2012). 9. Hamaguchi, K. et al. Astrophys. J. 695, L4–L9 (2009). 10. Moniez, M. Gen. Relativ. Gravit. 42, 2047–2074 (2010).
Space Carrier Vehicles by Oswald H. Lange and Richard J. Stein — The book begins with a useful conspectus of United States space launching missiles from the Juno 1 to the Saturn C-5: a list of their achievements is included, which shows the Thor-Agena B to be well in the lead, with 39 successful launches before the end of 1962 … Further subjects discussed are inertial guidance and control, the fabrication of the missiles (including an informative series of photographs of the Saturn vehicle under construction) … and, finally, the layout and construction of launching sites (with photographs of the Saturn launch complex at Cape Kennedy) … The book shows a bias in favour of German or American achievements: p.1 gives the impression that the first satellite was launched by the United States. From Nature 18 January 1964
100 Years Ago An article in Engineering for January 9 directs attention to the waning supply of petroleum. Although a continually greater supply of petroleum is being placed on the market, this increased output is secured only by sinking more wells and boring to a greater depth, showing that the surface supply is becoming exhausted. At the beginning of this century the wells touched 1100 ft., and to-day the average level of the oil may be placed at 2000 ft. — an ominously rapid rate of sinking … America, by reckless expenditure of her resources, has increased her annual output to 200 million barrels, yet the demand for oil for special purposes has become so great that the rise in price is considerable — so great, indeed, that competition with coal for ordinary purposes has become impossible. From Nature 15 January 1914
1 6 JA N UA RY 2 0 1 4 | VO L 5 0 5 | N AT U R E | 2 9 7
© 2014 Macmillan Publishers Limited. All rights reserved
that the ibises actively adjust their flapping pattern under different conditions. Although these findings are qualitatively consistent with theoretical predictions, many challenging questions remain. For example, how much energy do the birds actually save? The best existing evidence that V formations save a significant amount of energy is that pelicans have a lower heart rate and show reduced flapping frequency when flying in formation compared with when flying solo9. Accurate measurements of metabolic rate will be crucial for a more precise understanding of the underlying aerodynamics of formation flight and for greater insight into the ecology of bird migration. Do ibises and other birds instinctively flap in an efficient manner when flying in formation, or do they learn to adjust their body position and wing motion because it ‘feels’ easier? And if the strategy is so useful, why do many species of small migrating birds not fly in a V formation? Might the benefits of formation flying decrease with body size, or is the requisite control of body position and wing motion more difficult for smaller,
faster-flapping birds? Although our understanding of V formations has improved, there is still much to ponder when looking skyward on late summer days. ■ Florian T. Muijres and Michael H. Dickinson are in the Department of Biology, University of Washington, Seattle, Washington 98195-1800, USA. e-mail: [email protected] 1. Ning, S. A. Aircraft Drag Reduction Through Extended Formation Flight. PhD thesis, Stanford Univ. (2011). 2. Vachon, M. J., Ray, R., Walsh, K. & Ennix, K. in AIAA Atmos. Flight Mech. Conf. Exhib. Abstr. 2002-4491 (Am. Inst. Aeron. Astronautics, 2002); http://dx.doi. org/10.2514/6.2002-4491 3. Willis, D. J., Peraire, J. & Breuer, K. S. 25th AIAA Appl. Aerodynam. Conf. Abstr. 2007-4182 (2007); http://dx.doi.org/10.2514/6.2007-4182 4. Portugal, S. J. et al. Nature 505, 399–402 (2014). 5. Lissaman, P. B. S. & Lundry, J. L. J. Aircr. 5, 17–21 (1968). 6. Hummel, D. J. Theor. Biol. 104, 321–347 (1983). 7. Badgerow, J. & Hainsworth, F. J. Theor. Biol. 93, 41–52 (1981). 8. Maeng, J.-S., Park, J.-H., Jang, S.-M. & Han, S.-Y. J. Theor. Biol. 320, 76–85 (2013). 9. Weimerskirch, H., Martin, J., Clerquin, Y., Alexandre, P. & Jiraskova, S. Nature 413, 697–698 (2001).
ASTR O PH YSI CS
Black hole found orbiting a fast rotator Stars of spectral type ‘Be’ are often found with neutron stars or other evolved analogues, but a black-hole companion has never been spotted before. Optical emission from a black hole’s surroundings has given it away. See Letter p.378 M . V I R G I N I A M C S WA I N
S
tellar-mass black holes, which are formed from the gravitational collapse of massive stars, are unusually scarce in our Universe. It is not yet clear whether there are fewer of them than expected or whether they are just hard to find, but either way they are deserving of their exotic reputation. Therefore, the discovery by Casares et al.1, reported on page 378 of this issue, of a stellar-mass black hole orbiting around a star dubbed MWC 656 is like finding a needle in a haystack. This black hole does not emit X-ray radiation — as black holes are expected to do — so it could be the first sign of a large population of ‘quiescent’ black holes. MWC 656 is itself interesting because it is surrounded by a dense outflow from the star’s equator caused by a combination of its fast rotation (the projected rotational velo city2 is about 300 kilometres per second) and pulsations that can eject material from the equator3. The resulting circumstellar disk produces spectral emission lines from hydrogen and other elements, meriting its classification
2 9 6 | N AT U R E | VO L 5 0 5 | 1 6 JA N UA RY 2 0 1 4
© 2014 Macmillan Publishers Limited. All rights reserved
as a ‘B-emission’ or ‘Be’ star. Fast rotation is a requirement in the formation of Be stars, and they probably acquire their high angular momentum during mass transfer from a massive companion star that eventually explodes as a supernova. In fact, many Be stars are found with highly evolved companions, usually neutron stars that are remnants of the postsupernova massive stars4. A few Be stars have companion stars that have been stripped down to just their helium cores5, but such a hot object is ruled out in the case of MWC 656 because there is no significant ultraviolet-light contribution coming from anywhere other than the Be star. MWC 656 is the first Be star to have a black-hole companion detected (Fig. 1). Casares et al. have identified emission lines from helium plasma that is trapped in an accretion disk around the black hole, as well as emission from the disk around the Be star, that provide a robust measurement of the mass ratio between the star and the black hole. Such emission lines are notoriously difficult to measure: they are broad and often asymmetric, complicating the usual procedures used to
NEWS & VIEWS RESEARCH
Circumstellar disk
Be star
50 Years Ago
Black hole
Accretion disk
Figure 1 | A Be star with a black-hole companion. Casares et al.1 have detected a quiescent black hole circling a Be star by measuring optical emission from the black hole’s accretion disk and from the large disk around the star. As the black hole orbits the star, some material from the circumstellar disk is transferred to the accretion disk. However, the high angular momentum of material in the accretion disk inhibits it from falling into the black hole, so there will be a gap between the accretion disk and the black hole, and the black hole remains quiescent.
measure the line centres, but the authors have taken care to reduce any underlying systematic errors. Taken together with the mass of the Be star, which is about 10–16 solar masses, the measured ratio implies that the black hole has a mass of between 3.8 and 6.9 solar masses. In studies of stellar evolution, conventional wisdom tells us that stellar-mass black holes form during the collapse of the cores of very massive stars — those with masses more than 25 times that of the Sun6 — once the stars exhaust their fuel, and that the collapse is possibly accompanied by a supernova. The supernovae that massive stars (8–25 solar masses) undergo are expected to produce neutron star remnants instead. These massive stars tend to form within close groups of stars, so binary star systems are the norm, and triple and quadruple systems are not unusual. The catastrophe of a supernova in a binary has dramatic consequences: if more than half of the total-system mass is lost, or if ‘kick’ velocity from the explosion propels the newly formed supernova remnant with enough momentum, the remnant and companion star could fly off in opposite directions7. But if the companion star does remain gravitationally bound to the remnant, an X-ray binary is formed: the black hole or neutron star remnant interacts with the remaining star to produce X-ray emission. Theorists predict that stellar-mass black holes are abundant. If this is so, we should find them all over the Milky Way. Many of them ought to be bound in X-ray binaries, whereas others should be freely floating through space. There are probably tens of millions of massive stars in the Milky Way that could potentially collapse into black holes, but there are only about 50 stellar-mass black holes known with good confidence8. X-ray studies of young star-forming regions such as the Carina Nebula, which might contain at least a few recent supernovae products, have not found any black holes9. Even large sky surveys are coming up with little as they search for the subtle brightness variations
of stars whose light bends around a foreground black hole that passes in front of the star (an effect known as microlensing)10. MWC 656 presents a rare opportunity to study mass transfer, angular momentum and accretion-disk physics around a quiescent black hole. Casares et al. find a hint of a hotspot on the black hole’s accretion disk that suggests that mass is pulled away from the Be star’s disk, crashing into the accretion disk when the stars make their closest approach during their orbit around one another. The absence of X-ray emission from this system is evidence that material is not channelled into the black hole; rather, it must be retained in a holding pattern within the accretion disk. Gas in the outer regions of the Be star’s disk will have high angular momentum, which will be transferred to the accretion disk during the mass transfer. Without an efficient mechanism to remove this angular momentum, accretion will be suppressed and the black hole will remain quiet. If there exists a larger population of Be star– black-hole binaries, such quiescence is probably the rule, not the exception. Casares et al. have shown us a way to find them. ■ M. Virginia McSwain is in the Department of Physics, Lehigh University, Bethlehem, Pennsylvania 18015, USA. e-mail: [email protected] 1. Casares, J. et al. Nature 505, 378–381 (2014). 2. Williams, S. J. et al. Astrophys. J. 723, L93–L97 (2010). 3. McSwain, M. V., Huang, W., Gies, D. R., Grundstrom, E. D. & Townsend, R. H. D. Astrophys. J. 672, 590–603 (2008). 4. Liu, Q. Z., van Paradijs, J. & van den Heuvel, E. P. J. Astron. Astrophys. 455, 1165–1168 (2006). 5. Gies, D. R. et al. Astrophys. J. 493, 440–450 (1998). 6. Fryer, C. L. Astrophys. J. 522, 413–418 (1999). 7. Brandt, N. & Podsiadlowski, P. Mon. Not. R. Astron. Soc. 274, 461–484 (1995). 8. Belczynski, K., Wiktorowicz, G., Fryer, C. L., Holz, D. E. & Kalogera, V. Astrophys. J. 757, 91 (2012). 9. Hamaguchi, K. et al. Astrophys. J. 695, L4–L9 (2009). 10. Moniez, M. Gen. Relativ. Gravit. 42, 2047–2074 (2010).
Space Carrier Vehicles by Oswald H. Lange and Richard J. Stein — The book begins with a useful conspectus of United States space launching missiles from the Juno 1 to the Saturn C-5: a list of their achievements is included, which shows the Thor-Agena B to be well in the lead, with 39 successful launches before the end of 1962 … Further subjects discussed are inertial guidance and control, the fabrication of the missiles (including an informative series of photographs of the Saturn vehicle under construction) … and, finally, the layout and construction of launching sites (with photographs of the Saturn launch complex at Cape Kennedy) … The book shows a bias in favour of German or American achievements: p.1 gives the impression that the first satellite was launched by the United States. From Nature 18 January 1964
100 Years Ago An article in Engineering for January 9 directs attention to the waning supply of petroleum. Although a continually greater supply of petroleum is being placed on the market, this increased output is secured only by sinking more wells and boring to a greater depth, showing that the surface supply is becoming exhausted. At the beginning of this century the wells touched 1100 ft., and to-day the average level of the oil may be placed at 2000 ft. — an ominously rapid rate of sinking … America, by reckless expenditure of her resources, has increased her annual output to 200 million barrels, yet the demand for oil for special purposes has become so great that the rise in price is considerable — so great, indeed, that competition with coal for ordinary purposes has become impossible. From Nature 15 January 1914
1 6 JA N UA RY 2 0 1 4 | VO L 5 0 5 | N AT U R E | 2 9 7
© 2014 Macmillan Publishers Limited. All rights reserved
