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A Search for the Tritium Hyperfine Line
from Nearby Stars

Francisco Valdes

Central Computer Services, National Optical Astronomy Observations¹,
P.O. Box 26732, Tucson, Arizona 85726

and

Robert A. Freitas, Jr.

Xenology Research Institute,
8256 Scottsdale Drive, Sacramento, California 95828

Received July 12, 1985; revised October 28, 1985
Icarus 65, 152-157 (1986)
0019-1035/86 $3.00
Copyright © 1986 by Academic Press, Inc.

All rights of reproduction in any form reserved

¹ Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.

Note: This web version is derived from an earlier draft of the paper and may possibly differ in some substantial aspects from the final published paper.

A search for the tritium hyperfine line at 1516 MHz from 108 assorted astronomical objects, with emphasis on 53 nearby stars, was conducted in June 1983. All stars within 20 light-years visible from the 26-m telescope at Hat Creek Radio Observatory were examined using 256 4883-Hz channels. Twelve stars were also examined using 1024 76-Hz channels. The wideband- and narrowband-channel observations achieved sensitivities of 5-14 x 10^-21 W/m²/channel and 0.7-2 x 10^-24 W/m²/channel, respectively. No detections were made. The tritium frequency is highly attractive for SETI work because the isotope is cosmically rare and the tritium hyperfine line is centered in the SETI waterhole region of the terrestrial microwave window. In addition to beacon signals, tritium hyperfine emission may occur as a byproduct of extensive nuclear fusion energy production by extraterrestrial civilizations. © 1986 Academic Press, Inc.

INTRODUCTION

Conditions favorable to the development of life and intelligence may be widespread in the galaxy. There may exist a considerable number of extraterrestrial civilizations capable of interstellar communication. It has been argued (Billingham and Oliver, 1973; Morrison et al., 1977; Billingham and Pesek, 1979) that a civilization wishing to advertise its existence for purposes of initiating communication would employ an electromagnetic beacon. Operation on a single narrowband radio frequency against a quiet background would produce an obviously artificial signal and make most efficient use of available transmitter power.

This paper reports the results of a search for artificial radio emissions at a previously unexplored wavelength-the tritium hyperfine line. Tritium, like hydrogen, has a narrow, well-defined spectral line. The neutral tritium ground-state hyperfine transition line at 1516.701470 9064(16) MHz (Mathur et al., 1967) is centered in the SETI waterhole region of the terrestrial microwave window (Billingham and Oliver, 1973) which lies between the H and OH spectral lines. The natural abundance of neutral atomic tritium in the neighborhood of a late-type main sequence star should be negligible, approximating the local interstellar medium abundance of <10^-12 cm^-3 based on solar wind measurements (Fireman et al., 1976). Tritium is unimportant in advanced stellar evolution (Cujec and Fowler, 1980), and its 12.5-year β-decay half-life ensures its virtual natural absence in the neutral atomic state, giving an extraordinarily quiet background for beacon acquisition.

Observation of the tritium line may also be useful as a search for evidence for the by-products of a spacefaring industrial civilization (Freitas, 1985). A likely intermediate fusion product and nuclear fuel, tritium may be stored, and may escape via diffusion or spillage, as a diatomic gas at "habitable zone" temperatures producing a circumstellar cloud of neutral atoms plausibly observable over interstellar distances. A range of tritium line strengths have been estimated (Freitas, 1985) and lie marginally within the detection capabilities of the Hat Creek Radio Observatory instrument.

There is no possibility of confusion with the H 163α and H 162α recombination lines at 1504.646 MHz and 1532.520 MHz, respectively, nor with the two 4 He lines 163α and 162α at 1505.259 MHz and 1533.144 MHz. The 1516-MHz tritium line lies well outside the reserved radioastronomy bands (1400-1427 MHz, 1660-1670 MHz), major radio broadcasting bands allocated to aviation communications, aeronautical and maritime satellites (1542.5-1558.5 MHz), and space operations telemetry (1525-1535 MHz). A variety of small fixed and mobile allocations exist between 1435 and 1525 MHz for transmissions between fixed stations, land/coastal radar tracking systems, and so forth, but these potentially troublesome sources of RFI appear relatively unimportant for SETI work. The tritium line is a very attractive choice for an unambiguously artificial interstellar communication frequency, unique because detection alone virtually satisfies the artificiality criterion for SETI beacon signals.

There are no previous reports of searches of individual stars at the tritium hyperfine line. The only survey spanning the tritium line, by Kardashev and Gindilis in 1972 (Tarter, 1982) covering various frequencies between 1337 and 1863 MHz. used a dipole antenna to search the entire sky which would not be very sensitive to likely artificial sources of tritium hyperfine radiation. A tritium line sky survey also provides valuable new data in a specific important region of the radio spectrum which has not yet been extensively investigated, a useful check on astrophysical and nucleosynthesis theories.

OBSERVATIONS AND DISCUSSION

A search centered on the tritium hyperfine line for 108 assorted astronomical objects, with emphasis on 53 nearby stars, was conducted from 28 June to 10 July 1983 using the 26-m radiotelescope at the University of California's Hat Creek Radio Observatory, located about 270 miles north of the Berkeley, California, campus. The HCRO telescope has a 0.5^o half-power beamwidth and a system noise temperature of 100 K. We used a 1024-channel autocorrelator spectrometer configurable to different bandwidths and able to accept one or both linear polarizations. The pointing was checked by observation of strong Solar System sources, i.e., Jupiter and Saturn. The system noise temperature and flux calibrations were made from observations of the H 163α recombination line in the Orion Nebula. The computer control program automatically corrected the source radial velocities for the motion of the Earth. Observations were made by beam switching in cycles of 5 min on the source and 5 min off the source. Total integration time ranged between 0.3 and 3 hr.

Within 30 light-years of the Sun there are 86 stars, 53 of which are visible from HCRO. A search for tritium effluent from astroengineering operations, taking account of expected thermal line broadening at about 300K (mean habitable zone temperature), requires approximately 9-KHz channels. We also hoped to detect sufficiently strong artificial narrowband signals in such wide channels. The first part of the experiment thus consisted of a series of "wideband" observations of the nearest stars. The 1024-channel system was configured as four segments of 256 channels each, two segments for each linear polarization, with 4883-Hz (0.97 km/sec) channels for a total bandwidth of 1.25 MHz (247 km/sec). Stars within 20 light-years were examined, along with a broad sample of 55 different astrophysical objects including planets, nebulae, HII regions, giants and supergiants variable stars of different types, novae and supernovae remnants, various unusual stars, black hole candidates, globular clusters, the galactic center, external galaxies, and quasars. Though no natural tritium emission was expected from these latter sources, they were observed for a possible unexpected discovery. The large bandwidth of ± 124 km/sec undoubtedly encompasses the motion of many of the sources for which radial velocities were not readily available. Of course, natural emissions from the extragalactic sources whose radial velocities were unavailable would not be observed at the tritium line.

The second part of the experiment consisted of a series of relatively narrowband observations of the 12 most solar-like stars within 20 light-years of Earth. These observations used all 1024 channels in one linear polarization only with a total bandwidth of 78 kHz (15.4 km/sec) or 76.2 Hz/channel (0.015 km/sec/channel), the minimum readily available at HCRO. Good radial velocities were found for all of these stars.

Data were plotted as accumulated and examined at once for possible detections. All data sets indicating a >3σ event were reobserved at the next opportunity during the run. Tables I and II summarize the results of the tritium line observations. The (epoch 1950), the mean radial velocity tables are ordered by RA and contain the (when available), the total integration time, object identification, equatorial coordinates and the rms sensitivities in antenna temperature per channel and in the flux units of watts per square meter per channel.

TABLE I
Summary of Wideband (4883 Hz/Channel) Tritium Line Observations
Object	RA	Dec	VR	Int	TA	F x 10²⁴
	(1950)	(1950)	(km/sec)	(sec)	(K)	(W/m²/chan)
CD-37 15492	0:02	-37:26	23.6	4,800	0.025	6.4
G 158-27	0:04	-7:48	-22.0	3,000	0.029	7.4
G 34 AB	0:15	43:44	14.0	4,200	0.029	7.4
TYCHO	0:22	63:52	0.0	5,400	0.022	5.6
M31	0:40	41:00	-61.0	3,600	0.031	7.9
M32	0:42	40:36	21.0	3,000	0.029	7.4
ETA CAS	0:46	57:33	9.4	3,600	0.031	7.9
WOLF 287	0:46	5:09	158.5	4,200	0.025	6.4
SCULPTOR	0:58	-34:00	0.0	3,600	0.031	7.9
IC1613	1:02	1:42	-125.0	3,600	0.031	7.9
L725-327	1:10	-17:32	28.0	4,200	0.029	7.4
M33	1:31	30:24	2.0	11,400	0.016	4.1
L726-8 AB	1:36	-18:13	29.0	5,400	0.022	5.6
TAU CETI	1:42	-16:12	-16.2	6,600	0.023	5.9
L 1159-169	1:57	12:51	0.0	4,200	0.029	7.4
MIRA	2:17	-3:12	63.8	4,800	0.025	6.4
ALGOL AB	3:05	40:46	4.0	3,600	0.031	7.9
53 ARIETI	3:05	17:41	28.0	4,200	0.029	7.4
EPS ERI	3:31	-9:38	15.4	4,200	0.029	7.4
PLEIDES	3:44	23:58	0.0	1,200	0.053	13.6
RW TAU	4:01	28:00	-20.0	3,600	0.031	7.9
40 ERI ABC	4:13	-7.44	-42.4	4,200	0.029	7.4
T Tau TAU	4:19	19:25	24.6	4,200	0.029	7.4
STEIN 2051 A	4:26	58:33	0.0	6,000	0.022	5.6
RV TAU	4:44	26:06	30.0	2,400	0.038	9.7
CAPELLA	5:13	45:57	30.2	3,600	0.031	7.9
HD 36395	5:29	-3:41	10.9	4,200	0.029	7.4
ORION NEB	5:33	-5:25	0.0	4,200	0.029	7.4
ROSS 47	5:39	12:29	103.0	4,200	0.029	7.4
FU ORI	5:43	9:03	0.0	3,000	0.029	7.4
XI ORI	5:51	20:16	-13.5	3,000	0,029	7.4
ORI N MOL CLD	5:53	-1:50	10.0	3,000	0.029	7.4
LP 658-2	5:53	-4:08	11.0	7,800	0,019	4.9
-21 1377	6:08	-21:51	2.0	3,000	0.029	7.4
RED RECTANGLE	6:18	-10:37	0.0	3,000	0.029	7.4
ROSS 614 AB	6:27	-2:46	24.0	4,200	0.029	7.4
XMAS TREE CL	6:38	9:56	0.0	4,200	0,029	7.4
SIRIUS AB	6:43	-16:39	-7.6	3,000	0.029	7.4
WOLF 294	6:52	33:20	36.0	3,600	0.027	6.9
LUYTEN 1668	7:22	23:00	26.0	3,600	0.029	7.4
PROCYON AB	7:37	5:21	-3.2	4,200	0.029	7.4
ROSS 882	7:40	3:48	18.0	4,200	0.029	7.4
LUYTEN 674-15	8:10	-21:24	0.0	4,200	0.029	7.4
Z CAM	8:20	73:17	0.0	1,200	0.046	11.8
M44	8:38	19:52	33.0	3,600	0.031	7.9
+53 1320 AB	9:11	52:54	10.5	4,200	0.029	7.4
W URSA MAJ	9:40	56:11	-46.0	3,600	0.031	7.9
M82	9:52	69:56	388.0	4,200	0.029	7.4
G 1618	10:08	49:42	-27.0	5,400	0.025	6.4
BD + 20 2465	10:17	20:07	9.9	4,200	0.029	7.4
WOLF 359	10:54	7:16	13.0	3,000	0.029	7.4
LALANDE 21185	11:01	36:18	-86.5	5,400	0.023	5.9
WX U MAJ AB	11:03	43:47	64.0	4,200	0.029	7.4
AC +79 3883	11:45	78:58	-119.0	3,600	0.031	7.9
3C273	12:27	2:19	0.0	3,000	0.029	7.4
M87	12:28	12:40	1180.0	3,000	0.029	7.4
HZ 29	12:30	38:12	0.0	3,600	0.031	7.9
WOLF 424 AB	12:31	9:18	-5.0	3,600	0.031	7.9
COR COROLA	12:54	38:35	-3.3	4,200	0.029	7.4
W VIR	13:24	-3:07	-65.5	4.800	0.025	6.4
LALANDE 25372	13:41	15:26	15.0	4,800	0.025	6.4
UZ LB	15:30	-8:22	0.0	4,200	0.029	7.4
R COR BOR	15:47	28:19	24.8	3,600	0.031	7.9
T COR BOR	15:57	26:04	-29.0	3,600	0.031	7.9
M4	16:21	-26:24	65.0	3,000	0.029	7.4
ANTARES	16:26	-26:19	-3.2	4,200	0.029	7.4
BD-12 4523	16:28	-12:32	-13.0	3,600	0.053	13.6
36 OPH ABC	17:13	-26:32	-3.0	3,600	0.031	7.9
KEPLER	17:28	-21:26	0.0	3,000	0.034	8.7
BD +68 946	17:37	68:23	-17.0	2,400	0.035	9.0
SAG AE	17:43	-28:59	0.0	3,000	0.029	7.4
BARNARDSSTAR	17:55	4:33	-108.0	4,200	0.029	7.4
M8 Lagoon	18:02	-24:20	3.0	3,000	0.029	7.4
70 OPH AB	18:03	2:31	-7.2	7,800	0.021	5.4
CV SER	18:16	-11:39	10.0	4,800	0.025	6.4
59 1915 AB	18:42	59:33	1.0	3,600	0.031	7.9
NOVA AQU 1918	18:46	0:31	0.0	1,200	0.053	13.6
ROSS 154	18:47	-23:53	-4.0	3,000	0.029	7.4
BETA LYRA	18:48	33:18	-19.2	3,000	0.029	7.4
SS443	19:09	4:54	0.0	3,000	0.029	7.4
VAN BIESBROEC	19:14	5:06	34.0	4,200	0.029	7.4
PSR 0531 +21	19:20	21:47	0.0	3,000	0.029	7.4
SIGMA DRAC	19:32	69:35	26.7	3,600	0.027	6.9
NGC6822	19:42	-14:54	65.0	1,200	0.046	11.8
ALTAIR	19:48	8:44	-26.3	3,600	0.031	7.9
CHI CYG	19:49	32:47	-1.9	3,600	0,031	7.9
CYG XI	19:57	35:04	-13.0	3,000	0.029	7.4
CYG AE	19:58	40:35	0.0	3,600	0.027	6.9
HR 7703 AB	20:08	-36:14	-131.2	3,600	0.031	7.9
P CYGNI	20:16	37:53	-8.9	3,600	0.031	7.9
CYG X3	20:31	40:47	0.0	4,800	0.025	6.4
661 CYG AB	21:05	38:30	-64.3	7,800	0.019	4.9
NOVA CYG	21:10	47:57	0.0	3,600	0.031	7.9
LAC 8760	21:14	-39:04	23.0	3,600	0.031	7.9
BETA CEPHEUS	21:28	70:20	-8.2	3,600	0,031	7.9
SS CYG	21:41	43:21	-62.0	3,600	0.031	7.9
NGC7252	22:18	-24:56	4828.0	3,000	0.029	7.4
KRUGER 60	22:26	57:27	-24.0	7,200	0.021	5.4
HELICAL NEB	22:27	-21:06	-15.0	3,000	0.029	7.4
LUYTEN 789-6	22:36	-15:36	-60.0	3,00(1	0.034	8.7
BD +43 4305	22:45	44:05	-1.5	4,200	0.029	7.4
ROSS 780	22:51	-14:31	8.7	3,600	0.031	7.9
LAC 9352	23:03	-36:08	9.7	3,600	0.031	7.9
CAS AE	23:21	58:32	0.0	3,600	0.027	6.9
ROSS 248	23:39	43:55	-81.0	4,200	0.029	7.4
R AQU	23:41	-15:34	-22.0	3,600	0.031	7.9
BD +1 47719	23:47	2:08	-64.0	4,200	0.029	7.4
JUPITER	15:59	-19:44	0.0	1.200	0,046	11.8
SATURN	13:37	-8:20	0.0	1,200	0,046	11.8

TABLE II
Summary of Narrowband (76.2 Hz/Channel) Tritium Line Observations
Object	RA	Dec	VR	Int	TA	*F x 10²⁴*
	(1950)	(1950)	(km/sec)	(sec)	(K)	(W/m²/chan)
ETA CAS	0:46	57:33	9.4	3,000	0.47	1.9
TAU CETI	1:42	-16:12	-16.2	21,000	0.18	0.7
EPS ERI	3:31	-9:38	15.4	3,000	0.47	1.9
40 ERI ABC	4:13	-7.44	-42.4	3,000	0.47	1.9
LP 658-2	5:53	-4:08	11.0	3,000	0.47	1.9
PROCYON AB	7:37	5:21	-3.2	3,000	0.47	1.9
G 1618	10:08	49:42	-27.0	3,600	0.43	1.7
36 OPH ABC	17:13	-26:32	-3.0	3,000	0.47	1.9
70 OPH AB	18:03	2:31	-7.2	3,000	0.47	1.9
SIGMA DRAC	19:32	69:35	26.7	3,000	0.47	1.9
HR 7703 AB	20:08	-36:14	-131.2	3,000	0.47	1.9
661 CYG AB	21:05	38:30	-64.3	5,400	0.35	1.4

The detection sensitivity was found by computing the channel-to-channel instrumental rms for the central 60% of each linearly flattened spectrum. The rms obeyed the relationship: rms = (2.24 ± 0.19) x (bandwidth x integration time)^-1/2 for all the data; that is, for all bandwidths and for all integration times. The system noise temperature, normally 60K at the HI frequency, was determined from the H 163α recombination line observation of the Orion Nebula. Calibration with the observations of this line by Menon and Payne (1969) gave a noise temperature of 100 ± 20 K. The difference from the HI noise temperature is caused by mistuning of the receiver near the tritium frequency. Hence the rms antenna temperature sensitivity is given by:

TA_rms = 100 x 2.24 x [bandwidth (Hz) x integration time (sec)]^-1/2

and the corresponding flux sensitivity per channel is given by:

F_rms = TA_rms x k x [channel width (Hz)] / [0.5 x antenna aperture (m²)] where k is Boltzmann's constant and antenna aperture is 527 m² for the HCRO instrument.

Our results were completely negative -- there were no detections. Though the tables report sensitivity for the total integration time, some of the longer integration times actually consist of many shorter integrations. Each individual integration was examined for transient signals, but minimum integration times were typically 30 min. Transient signals of much shorter duration would have been undetectable with our observing methods. We should emphasize that this project was carried out using the standard facilities provided at HCRO for astronomical programs, with only a slight mistuning of the HI receiver to reach the tritium line.

The wideband- and narrowband-channel observations achieved sensitivities of 5-14 x 10^-24 W/m²/channel and 0.7-2 x 10^-24 W/m²/channel, respectively. The narrowband search was sensitive to artificial 19.8-cm tritium-line beacon signals that might have been broadcast from an 8 MW, 26-m antenna within 20 light-years of Earth, and could have detected a similar antenna broadcasting only 0.2 MW of power from Tau Ceti. These results are comparable to. previous SETI investigations near the 21-cm line.

ACKNOWLEDGMENTS

Observing time was kindly provided by the Radio Astronomy Laboratory of the University of California, Berkeley, California, which operates Hat Creek Radio Observatory. Travel funds were provided by Kitt Peak National Observatory, the American Astronomical Society Travel Grant Program, and the Xenology Research Institute.

REFERENCES

Billingham, J., and B. Oliver (1973). Project Cyclops: A Design Study Of a System for Detecting Extraterrestrial Intelligent Life, revised ed. NASA CR-114445.

Billingham, J., and R. Pesek (Eds.) (1979). Communication with Extraterrestrial Intelligence. Pergamon, Oxford.

Cujec, B., and W. A. Fowler (1980). Neglect of D, T, and ³He in advanced stellar evolution. Astrophys. J. 236, 658-660.

Fireman, E. L., J. DeFelice, and J. D'Amico (1976). The abundances of ³H and ¹⁴C in the solar wind. Earth Planet. Sci. Lett. 32, 185-190.

Freitas, R. A., Jr. (1985). Observable characteristics of extraterrestrial technological civilizations. J. Brit. Interplanet. Soc. .38, 106-112.

Mathur, B. S., S. B. Crampton, D. Kleppner, and N. F. Ramsey (1967). Hyperfine separation of tritium. Phys. Rev. 159,14-17.

Menon, T. K., and J. Payne (1969). Observations of 28 hydrogen na lines from the Orion Nebula. Astrophys. J. 3, L25-L27.

Morrison, P., J. Billingham, and J. Wolfe (Eds.) (1977). The Search for Extraterrestrial Intelligence, SETI. NASA SP-419.

Tarter, J. (1982). Searching for THEM: Interstellar communications. Astronomy 10, 6-22.

Created: July 26, 1998
Last Modified: April 30, 1999
HTML Editor: Robert J. Bradbury

Revised and corrected by Robert A. Freitas Jr., 19 November 2002