A detailed understanding of stars underpins much of astrophysics. Stars are the most fundamental astrophysical objects, they are the basic constituents of galaxies, and, to a large extent, the overall properties of the universe are controlled by their evolution. Stars are huge balls of gas, and turbulent motions within them generate sound waves that cause the majority of them to wobble and ring like bells. The measurement and analysis of these acoustic oscillations is the only way to peer into the interior of a star. SONG in New Mexico will acquire and assemble a telescope and spectrograph instrument that will be part of a world-wide network dedicated to `listening' to stars 24 hours per day. Constant monitoring is the only way to directly observe the workings of their interiors.

A tremendous amount of physics has been learned about the Sun from helioseismology, but the Sun provides only a single example of stellar structure at a single snapshot in evolution. Detailed asteroseismic observations of other stars will greatly extend the range of crucial interior physics that can be sampled and understood. Furthermore, precise knowledge of stars leads to better characterization of the planets that many of them host [1Huber, Daniel; Chaplin, William J.; Chontos, Ashley; Kjeldsen, Hans; Christensen-Dalsgaard, Jørgen; Bedding, Timothy R.; Ball, Warrick; Brahm, Rafael; Espinoza, Nestor; Henning, Thomas; Jordán, Andrés; Sarkis, Paula; Knudstrup, Emil; Albrecht, Simon; Grundahl, Frank; Fredslund Andersen, Mads; Pallé, Pere L.; Crossfield, Ian; Fulton, Benjamin; Howard, Andrew W.; Isaacson, Howard T.; Weiss, Lauren M.; Handberg, Rasmus; Lund, Mikkel N.; Serenelli, Aldo M.; Rørsted Mosumgaard, Jakob; Stokholm, Amalie; Bieryla, Allyson; Buchhave, Lars A.; Latham, David W.; Quinn, Samuel N.; Gaidos, Eric; Hirano, Teruyuki; Ricker, George R.; Vanderspek, Roland K.; Seager, Sara; Jenkins, Jon M.; Winn, Joshua N.; Antia, H. M.; Appourchaux, Thierry; Basu, Sarbani; Bell, Keaton J.; Benomar, Othman; Bonanno, Alfio; Buzasi, Derek L.; Campante, Tiago L.; Çelik Orhan, Z.; Corsaro, Enrico; Cunha, Margarida S.; Davies, Guy R.; Deheuvels, Sebastien; Grunblatt, Samuel K.; Hasanzadeh, Amir; Di Mauro, Maria Pia; Garc&\acute;ia, Rafael A.; Gaulme, Patrick; Girardi, Léo; Guzik, Joyce A.; Hon, Marc; Jiang, Chen; Kallinger, Thomas; Kawaler, Steven D.; Kuszlewicz, James S.; Lebreton, Yveline; Li, Tanda; Lucas, Miles; Lundkvist, Mia S.; Mann, Andrew W.; Mathis, Stéphane; Mathur, Savita; Mazumdar, Anwesh; Metcalfe, Travis S.; Miglio, Andrea; Monteiro, Mário J. P. F. G.; Mosser, Benoit; Noll, Anthony; Nsamba, Benard; Ong, Jia Mian Joel; Örtel, S.; Pereira, Filipe; Ranadive, Pritesh; Régulo, Clara; Rodrigues, Tha&\acute;ise S.; Roxburgh, Ian W.; Silva Aguirre, Victor; Smalley, Barry; Schofield, Mathew; Sousa, Sérgio G.; Stassun, Keivan G.; Stello, Dennis; Tayar, Jamie; White, Timothy R.; Verma, Kuldeep; Vrard, Mathieu; Yıldız, M.; Baker, David; Bazot, Michaël; Beichmann, Charles; Bergmann, Christoph; Bugnet, Lisa; Cale, Bryson; Carlino, Roberto; Cartwright, Scott M.; Christiansen, Jessie L.; Ciardi, David R.; Creevey, Orlagh; Dittmann, Jason A.; Do Nascimento, Jose-Dias, Jr.; Van Eylen, Vincent; Fürész, Gabor; Gagné, Jonathan; Gao, Peter; Gazeas, Kosmas; Giddens, Frank; Hall, Oliver J.; Hekker, Saskia; Ireland, Michael J.; Latouf, Natasha; LeBrun, Danny; Levine, Alan M.; Matzko, William; Natinsky, Eva; Page, Emma; Plavchan, Peter; Mansouri-Samani, Masoud; McCauliff, Sean; Mullally, Susan E.; Orenstein, Brendan; Garcia Soto, Aylin; Paegert, Martin; van Saders, Jennifer L.; Schnaible, Chloe; Soderblom, David R.; Szabó, Róbert; Tanner, Angelle; Tinney, C. G.; Teske, Johanna; Thomas, Alexandra; Trampedach, Regner; Wright, Duncan; Yuan, Thomas T.; Zohrabi, Farzaneh (2019): A Hot Saturn Orbiting an Oscillating Late Subgiant Discovered by TESS, \aj 157:245].

Correspondingly, the two broad science goals of the SONG network are to

1. perform asteroseismology to study stars at a level of detail similar to what can be obtained for the Sun with disk-integrated observations;
2. search for and characterize planets in orbit around other stars.

Enabling nearly-continuous, high-precision data on bright stars from a global network offers the opportunity to carry out many types of studies, such as:

• extend tests of stellar evolution models to other stars
• precise characterization of exoplanet host stars
• understand the effects and evolution of internal stellar rotation
• study the structure and age of low-metallicity and low-mass stars
• understand the dependence of the excitation of solar-like oscillations on stellar parameters
• test asteroseismic scaling relations with eclipsing binaries.

The past decade has seen the re-emergence of observational stellar astrophysics. Consider just three large-scale surveys, out of many: (1) the spectroscopic survey of the Milky Way with APOGEE [2Majewski, Steven R.; Wilson, John C.; Hearty, Fred; Schiavon, Ricardo R.; Skrutskie, Michael F. (2010): The Apache Point Observatory Galactic Evolution Experiment (APOGEE) in Sloan Digital Sky Survey III (SDSS-III), in Cunha, Katia; Spite, Monique; Barbuy, Beatriz (Ed.): Chemical Abundances in the Universe: Connecting First Stars to Planets, pp. 480-481, PUBLISHER, ADDRESS]; the astrometric survey of Gaia [3Gilmore, G.; Randich, S.; Asplund, M.; Binney, et al.; Team, Gaia-ESO Survey (2012): The Gaia-ESO Public Spectroscopic Survey, The Messenger 147:25-31]; and the photometric time-domain survey of Kepler [4Koch, David G.; Borucki, William J.; Basri, Gibor; Batalha, Natalie M.; Brown, Timothy M.; Caldwell, Douglas; Christensen-Dalsgaard, Jørgen; Cochran, William D.; DeVore, Edna; Dunham, Edward W.; Gautier, Thomas N., III; Geary, John C.; Gilliland, Ronald L.; Gould, Alan; Jenkins, Jon; Kondo, Yoji; Latham, David W.; Lissauer, Jack J.; Marcy, Geoffrey; Monet, David; Sasselov, Dimitar; Boss, Alan; Brownlee, Donald; Caldwell, John; Dupree, Andrea K.; Howell, Steve B.; Kjeldsen, Hans; Meibom, Søren; Morrison, David; Owen, Tobias; Reitsema, Harold; Tarter, Jill; Bryson, Stephen T.; Dotson, Jessie L.; Gazis, Paul; Haas, Michael R.; Kolodziejczak, Jeffrey; Rowe, Jason F.; Van Cleve, Jeffrey E.; Allen, Christopher; Chand rasekaran, Hema; Clarke, Bruce D.; Li, Jie; Quintana, Elisa V.; Tenenbaum, Peter; Twicken, Joseph D.; Wu, Hayley (2010): Kepler Mission Design, Realized Photometric Performance, and Early Science, \apjl 713:L79-L86]. The data from these experiments have increased, by orders of magnitude, the number of stars for which we now have knowledge of their composition, pulsation properties, masses, radii, ages, motions, positions, orbiting planets, multiplicity, etc. spanning across much of the Galaxy. These surveys have provided key insights into how the Milky Way was formed and has evolved, as well as new constraints on theories of stellar structure and evolution beyond what has been learned from star clusters. They have also provided targets that can be followed up with more detailed observations to probe poorly understood physics.

SONG focuses on asteroseismology, a unique tool that opens a window into stellar interiors from observations of stellar oscillations. Asteroseismology of stars displaying solar-like oscillations has also been revolutionized as an observational science in the past decade, due to a flood of high-precision photometry from the CoRoT, Kepler, and now TESS space missions [20Chaplin, William J.; Miglio, Andrea (2013): Asteroseismology of Solar-Type and Red-Giant Stars, \araa 51:353-392, 21Hekker, S. (2013): CoRoT and Kepler results: Solar-like oscillators, Advances in Space Research 52:1581-1592, 22Ricker, George R.; Winn, Joshua N.; Vanderspek, Roland; Latham, David W.; Bakos, Gáspár Á.; Bean, Jacob L.; Berta-Thompson, Zachory K.; Brown, Timothy M.; Buchhave, Lars; Butler, Nathaniel R.; Butler, R. Paul; Chaplin, William J.; Charbonneau, David; Christensen-Dalsgaard, Jørgen; Clampin, Mark; Deming, Drake; Doty, John; De Lee, Nathan; Dressing, Courtney; Dunham, Edward W.; Endl, Michael; Fressin, Francois; Ge, Jian; Henning, Thomas; Holman, Matthew J.; Howard, Andrew W.; Ida, Shigeru; Jenkins, Jon M.; Jernigan, Garrett; Johnson, John Asher; Kaltenegger, Lisa; Kawai, Nobuyuki; Kjeldsen, Hans; Laughlin, Gregory; Levine, Alan M.; Lin, Douglas; Lissauer, Jack J.; MacQueen, Phillip; Marcy, Geoffrey; McCullough, Peter R.; Morton, Timothy D.; Narita, Norio; Paegert, Martin; Palle, Enric; Pepe, Francesco; Pepper, Joshua; Quirrenbach, Andreas; Rinehart, Stephen A.; Sasselov, Dimitar; Sato, Bun'ei; Seager, Sara; Sozzetti, Alessandro; Stassun, Keivan G.; Sullivan, Peter; Szentgyorgyi, Andrew; Torres, Guillermo; Udry, Stephane; Villasenor, Joel (2015): Transiting Exoplanet Survey Satellite (TESS), Journal of Astronomical Telescopes, Instruments, and Systems 1:014003, 5Hekker, S.; Christensen-Dalsgaard, J. (2017): Giant star seismology, åpr 25:1]. For example, Kepler provided oscillation spectra derived from four years of continuous data for hundreds of main-sequence and tens of thousands of giant stars, comprising the largest existing dataset for seismology. These observations have enabled detailed and profound astrophysical studies [23Garcia, Rafael A.; Ballot, Jérôme (2019): Asteroseismology of solar-type stars, Living Reviews in Solar Physics 16:4, 24Jackiewicz, Jason (2021): Solar-Like Oscillators in the Kepler Era: A Review, Frontiers in Astronomy and Space Sciences 7:102].

However, ground-based spectroscopic measurements of solar-like oscillations still have a hugely important role to play. In this case, the oscillation signal is encoded in the radial-velocity fluctuations that can be measured from stellar spectra. These observations can be used to target specific stars of interest almost anywhere in the sky. Velocity data provide a higher signal-to-noise ratio than photometry because the stellar background signal from granulation is much lower in velocity than intensity compared to the oscillations [25Grundahl, F.; Kjeldsen, H.; Christensen-Dalsgaard, J.; Arentoft, T.; Frandsen, S. (2007): Stellar Oscillations Network Group, Communications in Asteroseismology 150:300, 26Stello, Dennis; Huber, Daniel; Sharma, Sanjib; Johnson, Jennifer; Lund, Mikkel N.; Handberg, Rasmus; Buzasi, Derek L.; Silva Aguirre, Victor; Chaplin, William J.; Miglio, Andrea; Pinsonneault, Marc; Basu, Sarbani; Bedding, Tim R.; Bland-Hawthorn, Joss; Casagrande, Luca; Davies, Guy; Elsworth, Yvonne; Garcia, Rafael A.; Mathur, Savita; Di Mauro, Maria Pia; Mosser, Benoit; Schneider, Donald P.; Serenelli, Aldo; Valentini, Marica (2015): Oscillating Red Giants Observed during Campaign 1 of the Kepler K2 Mission: New Prospects for Galactic Archaeology, \apjl 809:L3]. This property allows the determination of the lowest-frequency modes of a star, which are the modes most sensitive to the core and are largely unaffected by stellar near-surface layers. Low-frequency oscillations have longer lifetimes (experience less damping), allowing more precise frequency determination. They also couple with internal gravity waves in the cores of evolved stars, revealing deep stellar structure [27Bedding, Timothy R.; Mosser, Benoit; Huber, Daniel; Montalbán, Josefina; Beck, Paul; Christensen-Dalsgaard, Jørgen; Elsworth, Yvonne P.; Garc&\acute;ia, Rafael A.; Miglio, Andrea; Stello, Dennis; White, Timothy R.; De Ridder, Joris; Hekker, Saskia; Aerts, Conny; Barban, Caroline; Belkacem, Kevin; Broomhall, Anne-Marie; Brown, Timothy M.; Buzasi, Derek L.; Carrier, Fabien; Chaplin, William J.; di Mauro, Maria Pia; Dupret, Marc-Antoine; Frandsen, Søren; Gilliland, Ronald L.; Goupil, Marie-Jo; Jenkins, Jon M.; Kallinger, Thomas; Kawaler, Steven; Kjeldsen, Hans; Mathur, Savita; Noels, Arlette; Silva Aguirre, Victor; Ventura, Paolo (2011): Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars, \nat 471:608-611, 28Mosser, B.; Goupil, M. J.; Belkacem, K.; Michel, E.; Stello, D.; Marques, J. P.; Elsworth, Y.; Barban, C.; Beck, P. G.; Bedding, T. R.; De Ridder, J.; Garc&\acute;ia, R. A.; Hekker, S.; Kallinger, T.; Samadi, R.; Stumpe, M. C.; Barclay, T.; Burke, C. J. (2012): Probing the core structure and evolution of red giants using gravity-dominated mixed modes observed with Kepler, åp 540:A143]. The coupling of these modes provides observational access to the very uncertain mixing processes in stellar cores, which have profound effects on stellar evolution [5Hekker, S.; Christensen-Dalsgaard, J. (2017): Giant star seismology, åpr 25:1]. Finally, radial-velocity data also allow for the detection of L=3 modes, which are also crucial for probing stellar cores and yielding ages [29Cunha, M. S.; Metcalfe, T. S. (2007): Asteroseismic Signatures of Small Convective Cores, \apj 666:413-422, 14Grundahl, F.; Fredslund Andersen, M.; Christensen-Dalsgaard, J.; Antoci, V.; Kjeldsen, H.; Hand berg, R.; Houdek, G.; Bedding, T. R.; Pallé, P. L.; Jessen-Hansen, J.; Silva Aguirre, V.; White, T. R.; Frand sen, S.; Albrecht, S.; Andersen, M. I.; Arentoft, T.; Brogaard, K.; Chaplin, W. J.; Harpsøe, K.; Jørgensen, U. G.; Karovicova, I.; Karoff, C.; Kjærgaard Rasmussen, P.; Lund, M. N.; Sloth Lundkvist, M.; Skottfelt, J.; Norup Sørensen, A.; Tronsgaard, R.; Weiss, E. (2017): First Results from the Hertzsprung SONG Telescope: Asteroseismology of the G5 Subgiant Star \ensuremath\mu Herculis, \apj 836:142].

The analysis of radial-velocity modulations caused by solar-like oscillations requires ultra high-precision measurements, of the order of 1 m/s, with time series of several days to weeks, or often more. Temporal coverage also has a strong impact on seismic measurements, needing to be nearly continuous to avoid frequency aliases in the Fourier domain. Currently, very few instruments are capable of reaching the necessary precision, and these are in high demand, making it difficult to obtain enough telescope time to perform these critical asteroseismic studies. SONG overcomes these difficulties using a growing network of observatories dedicated to asteroseismology.

We focus on stars that pulsate like our Sun in what follows, both for simplicity and because the interpretation of the data is much more advanced than for other types of pulsators. Such solar-like oscillators need not be solar-type, main-sequence stars at all; for example, almost all red giants, such as the bright Aldebaran [6Farr, Will M.; Pope, Benjamin J. S.; Davies, Guy R.; North, Thomas S. H.; White, Timothy R.; Barrett, Jim W.; Miglio, Andrea; Lund, Mikkel N.; Antoci, Victoria; Fredslund Andersen, Mads; Grundahl, Frank; Huber, Daniel (2018): Aldebaran b\textquoterights Temperate Past Uncovered in Planet Search Data, \apjl 865:L20], display solar-like oscillations. They are the result of acoustic pressure waves that are excited to small, yet observable amplitudes by near-surface turbulent convection [7Goldreich, P.; Keeley, D. A. (1977): Solar seismology. II. The stochastic excitation of the solar p-modes by turbulent convection., \apj 212:243-251]. These stars, therefore, must have an outer convection zone, with effective (surface) temperature below about 7000K, corresponding to an upper mass of about 1.5 solar masses on the main sequence and spectral types later than mid-F. For evolved subgiants and giants, G, K, and M stars are most typical (and masses can exceed 1.5 solar masses).

The oscillations are global modes in a star, which distort the stellar surface with a spatial pattern that can usually be described by spherical harmonics, resulting in luminosity and radial-velocity variations. The figure shows 4 examples, where the red and blue denote the (highly exaggerated) distortion of the particular part of the surface, and white are nodes (no distortion). Cancellation effects due to the point-source nature of distant stars only allow for observations of the lowest spherical harmonic degrees (L=0 - 3). In the figure, the top 2 animations are for L=1 and L=3, which would be observable. The other two (L=6 and L=10, would not be). Power spectra of a time series of a solar-like oscillator show a comb-like structure of peaks within a broad acoustic mode envelope that has a maximum amplitude at some temporal frequency. This can range from about 20 μHz (half a day period) for evolved giants to a few thousand μHz (periods of minutes) for dwarfs. The comb pattern has peaks that are evenly spaced in frequency, whereby frequency differences between modes of consecutive radial order n and the same spherical degree L are known as the large frequency spacing.

These observed modal properties are often interpreted in terms of the asymptotic theory of stellar oscillations [8Tassoul, M. (1980): Asymptotic approximations for stellar nonradial pulsations., \apjs 43:469-490]. In this case, the large frequency spacing is related to the sound crossing time of an acoustic wave across the star, and therefore scales with the mean density. An empirically-motivated relationship connects the frequenc of maximum power with the surface gravity and effective temperature [9Brown, Timothy M.; Gilliland, Ronald L.; Noyes, Robert W.; Ramsey, Lawrence W. (1991): Detection of Possible p-Mode Oscillations on Procyon, \apj 368:599]. When these two relations are combined, scaling relations for a star's mass and radius can be derived [10Kjeldsen, H.; Bedding, T. R. (1995): Amplitudes of stellar oscillations: the implications for asteroseismology., åp 293:87-106]

Other types of modes provide even more powerful diagnostics of interior conditions. In subsurface convectively-stable regions of stars, such as the cores of red giants, low-frequency gravity modes are excited. While they are not visible at the surface, they can interact and `mix' with acoustic modes and impart new information in the power spectrum. New frequency spacings appear, which can be used as diganostics of the core region, capable of providing the evolutionary state or inferring the core rotation rate.

All of the above may be considered as a broad application of asteroseismic analysis of global observables. SONG will go much further than applying the scaling relations. It will provide very precise individual mode frequencies for more modes than space photometry can, including mixed modes, L=3 modes, and modes split into multiplets by internal rotation. In fact, the relative amplitudes of the frequency multiplets can be used to measure the inclination of the stellar rotation axis [11Gizon, L.; Solanki, S. K. (2003): Determining the Inclination of the Rotation Axis of a Sun-like Star, \apj 589:1009-1019]. In any case, the measured frequencies can then be confronted with the frequencies predicted from sophisticated numerical models. The result is a deeper understanding of all of the interior stellar properties and evolutionary state of stars.

Asteroseismology is a time-domain science and data analysis is carried out in the temporal frequency (Fourier) domain. Analysis is most successful when the power spectrum of the time series shows oscillation modes at distinct frequencies that can be identified (guided by models or empirical relations) according to their radial order and spherical harmonic degree. This requires sufficient duration to resolve the modes in frequency space, as well as ample sampling to capture the highest frequencies: both of these criteria are straightforward to meet in practice, even from a single location.

However, regular gaps in the data (such as from the day/night cycle) cannot be overcome by simply observing longer or more often, and lead to a sampling (window) function that is not optimal. The Fourier transform of this function is known as the spectral window. The power spectrum of the target star is thus effectively a convolution of this spectral window and the true underlying oscillation spectrum. For gapped data, the spectral window introduces alias frequencies into the power spectrum that do not correspond to real frequencies, wreaking havoc with the analysis. That is why, before SONG, solar-like oscillations had only been detected in a handful of stars from the ground using ad-hoc networks or large telescopes, and Herculean analysis efforts [30Bedding, Timothy R.; Kjeldsen, Hans; Arentoft, Torben; Bouchy, Francois; Brandbyge, Jacob; Brewer, Brendon J.; Butler, R. Paul; Christensen-Dalsgaard, Jørgen; Dall, Thomas; Frandsen, Søren; Karoff, Christoffer; Kiss, László L.; Monteiro, Mario J. P. F. G.; Pijpers, Frank P.; Teixeira, Teresa C.; Tinney, C. G.; Baldry, Ivan K.; Carrier, Fabien; O'Toole, Simon J. (2007): Solar-like Oscillations in the G2 Subgiant \ensuremath\beta Hydri from Dual-Site Observations, \apj 663:1315-1324, 31Arentoft, Torben; Kjeldsen, Hans; Bedding, Timothy R.; Bazot, Michaël; Christensen-Dalsgaard, Jørgen; Dall, Thomas H.; Karoff, Christoffer; Carrier, Fabien; Eggenberger, Patrick; Sosnowska, Danuta; Wittenmyer, Robert A.; Endl, Michael; Metcalfe, Travis S.; Hekker, Saskia; Reffert, Sabine; Butler, R. Paul; Bruntt, Hans; Kiss, László L.; O'Toole, Simon J.; Kambe, Eiji; Ando, Hiroyasu; Izumiura, Hideyuki; Sato, Bun'ei; Hartmann, Michael; Hatzes, Artie; Bouchy, Francois; Mosser, Benoit; Appourchaux, Thierry; Barban, Caroline; Berthomieu, Gabrielle; Garcia, Rafael A.; Michel, Eric; Provost, Janine; Turck-Chièze, Sylvaine; Martić, Milena; Lebrun, Jean-Claude; Schmitt, Jerome; Bertaux, Jean-Loup; Bonanno, Alfio; Benatti, Serena; Claudi, Riccardo U.; Cosentino, Rosario; Leccia, Silvio; Frandsen, Søren; Brogaard, Karsten; Glowienka, Lars; Grundahl, Frank; Stempels, Eric (2008): A Multisite Campaign to Measure Solar-like Oscillations in Procyon. I. Observations, Data Reduction, and Slow Variations, \apj 687:1180-1190, 32Bedding, T. R.; Kjeldsen, H. (2008): Asteroseismology from Solar-Like Oscillations, in van Belle, G. (Ed.): 14th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun, pp. 21, PUBLISHER, ADDRESS].

The figure above demonstrates this issue by contrasting observations from idealized two- or three-site networks. Space-based Sun-as-a-star velocity data (for which we have very long, uninterrupted time series), mimicking a distant pulsating star, were used to construct a one-month long time series with a one-minute sampling. Regular gaps of 8 hours were introduced every 24 hours to simulate a network of two sites, while no regular gaps are present in the idealized three-site network. The resulting power spectra and spectral windows are shown in the figure. Since the two-site spectral window has aliasing frequencies at 1 cycle/day =11.57 μHz and its overtones (right panel), each peak in the corresponding power spectrum is surrounded by false peaks. These peaks can interfere with neighboring modes, causing contamination [12Arentoft, T.; Tingley, B.; Christensen-Dalsgaard, J.; Kjeldsen, H.; White, T. R.; Grundahl, F. (2014): Benefits of multiple sites for asteroseismic detections, \mnras 437:1318-1328]. This is clearly evident in the inset panel. Furthermore, the amplitudes of the modes are altered by the contamination.

The introduction of false frequencies that overlap and interfere with real ones can be absolutely detrimental to the interpretation and analysis of the oscillation spectra – this is the prime motivation for establishing a global network [13Jain, Kiran; Tripathy, Sushanta C.; Hill, Frank; Pevtsov, Alexei A. (2021): Continuous Solar Observations from the Ground-Assessing Duty Cycle from GONG Observations, \pasp 133:105001]. We recognize that continuous coverage for long periods will still be limited by weather conditions, but the presence of random gaps has significantly less effect than the regular gaps that arise from a lack of full longitudinal coverage.

The SONG-Tenerife node has been operating since 2014. While single-site observations are not ideal due to the day/night gaps, there are certain stars with just the right oscillation properties that allow for unambiguous asteroseismic determinations [12Arentoft, T.; Tingley, B.; Christensen-Dalsgaard, J.; Kjeldsen, H.; White, T. R.; Grundahl, F. (2014): Benefits of multiple sites for asteroseismic detections, \mnras 437:1318-1328]. The success so far of SONG from just one site has been remarkable, showing that the instrumentation we propose to duplicate is a proven technology and fully achieves the expected precision.

The best observed SONG target is the G5 subgiant μ Hercules (HD 161797). This M_V=3.82 star is slightly more massive than the Sun and has been observed for over 215 nights with 120 s exposures. The average radial-velocity precision is 1.4 m/s, per spectrum. At least 49 acoustic modes, including five L=3 modes have been identified in this data set [14Grundahl, F.; Fredslund Andersen, M.; Christensen-Dalsgaard, J.; Antoci, V.; Kjeldsen, H.; Hand berg, R.; Houdek, G.; Bedding, T. R.; Pallé, P. L.; Jessen-Hansen, J.; Silva Aguirre, V.; White, T. R.; Frand sen, S.; Albrecht, S.; Andersen, M. I.; Arentoft, T.; Brogaard, K.; Chaplin, W. J.; Harpsøe, K.; Jørgensen, U. G.; Karovicova, I.; Karoff, C.; Kjærgaard Rasmussen, P.; Lund, M. N.; Sloth Lundkvist, M.; Skottfelt, J.; Norup Sørensen, A.; Tronsgaard, R.; Weiss, E. (2017): First Results from the Hertzsprung SONG Telescope: Asteroseismology of the G5 Subgiant Star \ensuremath\mu Herculis, \apj 836:142]. Some of the L=1 modes are mixed with gravity waves, making this the most robust set of mixed modes ever observed from the ground. Only the Sun has provided as much precise seismic information as this star.

μ Her is also orbited by three M dwarfs. Seismology with SONG data using the rotational frequency splittings has shown that μ Her's rotation axis is aligned with the system's orbital axis, making it one of the few obliquity measurements available for a multiple-star system. [15Li, Tanda; Bedding, Timothy R.; Kjeldsen, Hans; Stello, Dennis; Christensen-Dalsgaard, Jørgen; Deng, Licai (2019): Asteroseismic modelling of the subgiant \ensuremath\mu Herculis using SONG data: lifting the degeneracy between age and model input parameters, \mnras 483:780-789] extended the seismic modeling of the SONG data for μ Her and determined its mass and age to better than 10% precision. More interestingly, they were able to lift an important degeneracy between age, composition, and mixing length by tightly constraining the initial He abundance with the precise frequencies from SONG.

The baseline over which μ Her has been observed is approaching 8 years. And, like the Sun [16Basu, Sarbani (2016): Global seismology of the Sun, Living Reviews in Solar Physics 13:2], its individual mode frequencies are beginning to show long-term trends indicative of a magnetic activity cycle. The long lifetime expected for SONG opens up the remarkable opportunity to study stellar cycles seismically, measuring time variations of internal properties, and thus adding to the richness of the data set for future research directions.

There are many other recent successes from the single SONG node that showcase its potential for transformative stellar astrophysics when the network is operational. Several examples are: a study of planet-hosting red giants to obtain accurate masses to understand the main-sequence progenitors, and thus, the stellar environment in which the planets formed [17Stello, Dennis; Zinn, Joel; Elsworth, Yvonne; Garcia, Rafael A.; Kallinger, Thomas; Mathur, Savita; Mosser, Benoit; Sharma, Sanjib; Chaplin, William J.; Davies, Guy; Huber, Daniel; Jones, Caitlin D.; Miglio, Andrea; Silva Aguirre, Victor (2017): The K2 Galactic Archaeology Program Data Release. I. Asteroseismic Results from Campaign 1, \apj 835:83]; an asteroseismic mass of a planet-hosting red giant in the Hyades cluster that leads to a revised minimum mass for the companion planet [18Arentoft, T.; Grundahl, F.; White, T. R.; Slumstrup, D.; Handberg, R.; Lund, M. N.; Brogaard, K.; Andersen, M. F.; Silva Aguirre, V.; Zhang, C.; Chen, X.; Yan, Z.; Pope, B. J. S.; Huber, D.; Kjeldsen, H.; Christensen-Dalsgaard, J.; Jessen-Hansen, J.; Antoci, V.; Frandsen, S.; Bedding, T. R.; Pallé, P. L.; Garcia, R. A.; Deng, L.; Hon, M.; Stello, D.; Jørgensen, U. G. (2019): Asteroseismology of the Hyades red giant and planet host \ensuremathın Tauri, åp 622:A190]; and a test of the seismic scaling relations in a star over 200 times more metal poor than the Sun, which was not known to be a pulsator until SONG came along [19Creevey, O.; Grundahl, F.; Thévenin, F.; Corsaro, E.; Pallé, P. L.; Salabert, D.; Pichon, B.; Collet, R.; Bigot, L.; Antoci, V.; Andersen, M. F. (2019): First detection of oscillations in the Halo giant HD 122563: Validation of seismic scaling relations and new parameters, åp 625:A33].

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