Abstract
1I/'Oumuamua is the first confirmed interstellar body in our solar system. Here we report on observations of 'Oumuamua made with the Spitzer Space Telescope on 2017 November 21–22 (UT). We integrated for 30.2 hr at 4.5
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1. Introduction
'Oumuamua (1I/2017 U1) was discovered on 2017 October 18. One week later it was announced that 'Oumuamua's orbit was unbound (Bacci et al. 2017) and that this was the first ever discovered interstellar body—an object that originated outside our solar system.
It has long been thought that comets and asteroids exist in other planetary systems. Most current models of our own solar system suggest that today's small bodies are leftovers from the era of planet formation (e.g., Dones et al. 2015), implying that other planetary systems also produced comet and/or asteroid populations. Until now, it has been impossible to connect our own local small-body populations to the large, but unresolved, groups of comets and asteroids found in exoplanetary circumstellar disks (e.g., Lisse et al. 2007, 2017).
'Oumuamua was the subject of an intense, though brief, observing campaign (Bannister et al. 2017; Jewitt et al. 2017; Knight et al. 2017; Masiero 2017; Meech et al. 2017; Ye et al. 2017; Belton et al. 2018; Bolin et al. 2018; Drahus et al. 2018; Fitzsimmons et al. 2018; Fraser et al. 2018; Micheli et al. 2018). In summary, 'Oumuamua has a red, featureless visible/near-infrared spectral slope, no directly-detected emission of gas or dust (though activity may be required to explain the presence of non-gravitational perturbations affecting its motion), a very elongated shape, and an excited rotation state. The color, spectral slope, density, and lack of apparent activity all suggest something like a D-type (primitive) asteroid, though the implied low-level activity points to a comet-like body. (The shape and rotation state do not particularly imply any specific analog in our solar system.) Assuming the object to have asteroidal density, McNeill et al. (2018) showed that no significant cohesive strength is required for 'Oumuamua to resist rotational fission, but even assuming a comet-like bulk density of 0.5 g cm−3 we find that a trivial cohesive strength of only 1 ± 1 Pa is required.
The existence of 'Oumuamua has implications for its formation and origin and on the small-body populations in other planetary systems (Trilling et al. 2017; Ćuk 2018; Do et al. 2018; Feng & Jones 2018; Gaidos 2018; Jackson et al. 2018; Katz 2018; Raymond et al. 2018a, 2018b; Zwart et al. 2018). Overall, these formation models generally prefer a comet-like body for interstellar interlopers.
As part of the observational campaign carried out before 'Oumuamua became too faint, we observed this body with the Spitzer Space Telescope. Spitzer observations offered the best possibility to determine the diameter and albedo of this object by measuring its emitted thermal infrared radiation as our team has done for thousands of Near Earth Objects (NEOs; Trilling et al. 2010, 2016.)
Here we present the results of our Spitzer observations. We did not convincingly detect 'Oumuamua and are left with an upper limit on its flux that corresponds to an upper limit on diameter and a lower limit on albedo. In Section 2, we present our observational approach and data reduction steps, details of the ephemeris and uncertainty calculations, and our observational results. In Section 3, we present our thermal modeling and the resulting limits on diameter and albedo, which strongly depend on choice of model parameters. We discuss our model results and search for activity in Section 4.
2. Observations and Results
2.1. Observations and Data Reduction
Observations were obtained with Spitzer/IRAC (Fazio et al. 2004) as part of DDT program 13249. Seven Astronomical Observing Requests (AORs) were used, six of ∼5 hr duration with 166 × 100 s frames, and a final 2.9 hr (clock time) AOR with 94 × 100 s frames, for a total of 1090 frames and 30.2 hr on-source frame time (acquired over 33 hr of clock time). The observations were divided in this way because of limits in the number of commands and data allowed in a single AOR. The data were taken with the "Moving Single" target mode with Full Array readouts, using a small cycling dither pattern. Two frames were taken at each dither position, to reduce the overhead of moving after each frame. Images were obtained in both arrays, but only the 4.5
'Oumuamua was discovered on 2017 October 19 (and announced as an interstellar body on 2017 October 25; Bacci et al. 2017), but because of the constraints of the Spitzer observability zone, the earliest that the Spitzer observations could begin was late on November 20 (Figure 1). The ephemeris used to develop the original Spitzer observation sequence was based on ground-based astrometric data through the end of October and had a prediction uncertainty larger than the Spitzer FOV. On November 9, the Magdalena Ridge Observatory collected additional ground-based observations, which we used together with preliminary high-precision astrometry from Micheli et al. (2018) to refine the orbit of 'Oumuamua. We found that the revised predicted positions could potentially put the object very close to or off the edge of the array for many frames in the AORs constructed with the original ephemeris. The Spitzer Science Center (SSC) was able to replan the observations with the latest orbit information. The first AOR began executing at 2017 November 21 10:13:26 UT, and the last AOR completed at 2017 November 22 18:52:06 UT; this is around 2.5 months after 'Oumuamua's perihelion passage. The average heliocentric distance of 'Oumuamua during the observations was 2.0 au and the average Spitzer-centric distance was 1.8 au; the average phase angle was around 31° (Figure 1). This geometry changed only very slightly during the 33 hr of clock time needed to carry out these observations. The rate of motion on-sky in these observations was around 68 arcsec/hr.
The data reduction method used was similar to that described in Mommert et al. (2014b). A mosaic of the field was constructed from the data set itself and then subtracted from the individual basic calibrated data (BCD) frames. After subtraction of the background mosaic, residual background sources and bright cosmic ray artifacts were masked in the individual BCDs before being mosaicked in the reference frame of the moving object.
2.2. Ephemeris and Positional Uncertainties
Micheli et al. (2018) later collected ground-based and Hubble Space Telescope astrometry of 'Oumuamua, eventually extending the observational arc through 2018 January 2. Based on this longer sampling of the trajectory, they reported a 30
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Standard image High-resolution image2.3. Observational Results
Our final mosaic is shown in Figure 3 along with the predicted location of 'Oumuamua. There are no bright coherent sources in this image, so we conclude that we did not confidently detect the source. The 1
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Standard image High-resolution imageThere are several ∼2
There is no significant vignetting in the 4.5
3. Thermal Modeling and Interpretation of the Non-detection
We rule out any detections of 'Oumuamua at greater than 3
We simulate the expected brightness of 'Oumuamua in Spitzer IRAC Channel 2 in order to interpret our upper limit detection. Using the Near-Earth Asteroid Thermal Model (NEATM; Harris 1998), we estimate the target's brightness as a function of its absolute magnitude HV and a range in geometric albedo (0.01 ≤ pV ≤ 1.0). Since the physical properties of 'Oumuamua are unknown, we consider a range of values for the thermal infrared beaming parameter: 0.8 ≤
Figure 4 shows the distribution of predicted IRAC Channel 2 flux densities for the three different beaming parameters
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Standard image High-resolution imageDownload figure:
Standard image High-resolution image4. Discussion
4.1. Search for Activity
Based on the discovery of non-gravitational accelerations acting upon the orbit of 'Oumuamua (Micheli et al. 2018), we investigate the possibility of dust and gas activity in this object during our observations. Our non-detection enables the placement of upper limits on the production rates of dust, as well as CO and CO2 gas; we are unable to constrain the production of other gas species. We use the same formalism that we used in detecting cometary behavior in the NEO Don Quixote (Mommert et al. 2014c)—Bauer et al. (2015) used similar approaches with WISE data—and our measured 3
This CO upper limit is much lower than the Micheli et al. (2018) value of 4.5 × 1025 molecules s−1 (the most sensitive search in the literature) and implies that the outgassing from 'Oumuamua cannot have CO (or, presumably, CO2) as a significant component, though the Micheli et al. (2018) CO production rate assumes a relative large body and albedo of 4%. If 'Oumuamua's size were 10–20 times smaller than the Micheli et al. (2018) diameter of 220 m, then the amount of CO outgassing at the upper limit would produce sufficient acceleration. However, an effective spherical diameter of 10–20 m would require an unacceptably high albedo and unacceptably low
4.2. Uncertainties
Our analysis of 'Oumuamua's physical properties is based on a measured flux density upper limit and thermal modeling performed with the NEATM. This model has been specifically designed for use on near-Earth asteroid observations and has been shown to be reasonably accurate over a wide range of cases (Harris et al. 2011; Mommert et al. 2018). It is applicable to thermal emission from any airless body and has been used extensively for comet nuclei as well (Lisse et al. 2005, 2009; Fernandez et al. 2013). A more sophisticated thermophysical model (Mommert et al. 2014a, 2014b, 2018) is not appropriate here due to the lack of information on the target (e.g., spin pole orientation and complex rotation state; shape is somewhat known but not uniquely so) and the upper-limit nature of the flux density measurement.
'Oumuamua is known to have a high-amplitude light curve (e.g., Jewitt et al. 2017; Knight et al. 2017; Meech et al. 2017; Bolin et al. 2018; Micheli et al. 2018) with, most likely, a period of 6–8 hr. Since our observations spanned 33 hr (clock time), any light curve effects are smoothed out, and we observe only the average flux. Furthermore, even though the Spitzer viewing geometry of 'Oumuamua is very different from that seen by observatories on and near the Earth, because 'Oumuamua is in an excited rotation state (Belton et al. 2018; Drahus et al. 2018; Fraser et al. 2018), we likely observed the same time-averaged projected surface area that would have been seen from Earth. Furthermore, the light curves presented in Belton et al. (2018) are not sinusoidal but rather have broad maxima and narrow minima, so our 33 hr integration is likely not corrupted by faint epochs in the light curve. Even in the case of a 55 hr period, one possible solution suggested by Belton et al. (2018), our observations span a significant fraction of the entire rotation and therefore included something close to the time-averaged cross-sectional area, except in the case of a pathological orientation.
We do not include uncertainties on the ratio of infrared to optical reflectances, as the impact of this ratio barely affects the overall results of this study, especially in the light of the large uncertainties on the beaming parameter
We investigate the applicability of NEATM for this study given the high aspect ratio of 'Oumuamua (Meech et al. 2017; McNeill et al. 2018) and the assumption of sphericity in NEATM. For this purpose, we use an asteroid thermophysical model (Mommert et al. 2014a, 2014b, 2018) to derive the thermal and reflected solar flux density of the body, assuming both a highly elongated shape and a highly oblate shape (following Belton et al. 2018). Based on McNeill et al. (2018), we assume a triaxial ellipsoidal shape with semimajor axes 6:1:1 for the highly elongated shape and 1:: for the highly oblate shape, both in arbitrary units. Furthermore, we use the geometry during our Spitzer observations, the period (7.34 hr) derived by Meech et al. (2017), and assume a geometric albedo of 0.03 (in agreement with our NEATM-derived lower limit) and typical small-body values for thermal inertia and surface roughness. We simulate the flux density observed at Spitzer over one-quarter of the target's rotation and derive the average flux density, which is the quantity measured in our observations by combining all available data. Finally, we form the ratio of the average flux density derived for a spherical body (NEATM assumption) to the average flux density derived from the elongated shape and oblate shape models. Deriving this ratio minimizes the effects of the choice of the geometric albedo, surface roughness, and thermal inertia used in the simulation.
We find that this flux density ratio varies as a function of the target's spin axis latitude (as a proxy for the aspect angle of our observations). In the case of the elongated shape, a spin axis latitude of 90° (equator-on view), the ratio is 1, and rotational effects are averaged out during our observations. The ratio decreases to 0.5 for spin axis latitudes approaching 0 (pole-on view), which represents an extreme case. In the case of the oblate shape, we find ratios of around [0.5, 1, 2] for
4.3. Discussion of Possible Solutions
4.3.1. Low-albedo Solution
Since 'Oumuamua is in an excited rotation state, absorption of solar energy could be significantly more uniform around the surface than for rotation around a single axis. This implies the temperature distribution would be smoother than a single axis rotator, requiring a higher
Under the conservative assumption of
4.3.2. Mid-range Albedo Solution
The default approach used in our Spitzer NEO program is to derive
4.3.3. High-albedo Solution
Finally, a lower
If 'Oumuamua has a high albedo then its inferred size (98 meter diameter) is substantially smaller than the 220 meter diameter that was assumed by Micheli et al. (2018), and its mass is smaller by the cubed ratio of these solutions ((98/220)3 = 1/11). With a smaller mass, greater acceleration is produced for a given force (i.e., outgassing). However, force is proportional to the production rate, and the CO production rate derived here is 104 times less than that used by Micheli et al. (2018) to explain the measured astrometry. This rules out the the possibility that CO or CO2 outgassing was responsible for the non-gravitational acceleration that Micheli et al. (2018) detected. However, we cannot put constraints on outgassing of water ice, which is the other main volatile ice found in comets, using our data (see below).
4.4. Summary of Results and a Possible Interpretation
There are several possible interpretations of our results, as follows. We note that in all cases, given our upper limit on CO and CO2 production rates, some other gas species (e.g., water) must also have been emitted to explain the non-gravitational acceleration observed by Micheli et al. (2018). We can place no constraint on these other gas species.
(1) 'Oumuamua could have a high
In conclusion, there is no simple asteroidal or cometary physical model that agrees with expectations and previous work (including non-gravitational acceleration) and our results for all of
We assume that 'Oumuamua is outgassing 9 × 1022 molecules of CO2 per second (see above). In the high-albedo case, the effective spherical diameter of 'Oumuamua is around 98 m, and the surface area is therefore around 3 × 104 m2 (taking 49 m as the radius of the equivalent sphere). If we require a uniform surface layer that is 10
The density of CO2 ice is approximately 1.5 g cm−3. The mass required to create a surface layer of 0.3 m3 is therefore 4.5 × 105 g. We calculate the number of CO2 molecules required as
which is around 6 × 1027 molecules of CO2. Given the CO2 upper limit of 9 × 1022 molecules s−1 derived above, depositing 6 × 1027 molecules of CO2 requires only around 67000 s, or around 0.8 days—far less than the few weeks of 'Oumuamua's perihelion passage time. Thus, even if the efficiency of this process is small, it is still quite plausible that a low level of activity—induced by solar heating of a near-subsurface CO2 reservoir—could produce enough material to coat the surface with bright, fresh CO2 and increase the albedo to the relatively high value required in our high-albedo case.
Alternately, heating of water ice into gas could present a plausible scenario. Cometary dust to gas ratios are typically around 5:1, so our dust emission upper limit of 9 kg s−1 corresponds to 1.8 kg s−1 as an upper limit for gas emission. If we assume that all of this gas emission is in H2O, then we find
which is around 6 × 1025 molecules s−1, enough to produce the non-gravitational accelerations reported by Micheli et al. (2018).
CO+CO2 ice in typical solar system comets is around 15% of the water abundance. Here our limits imply around 0.15% for this ratio—a factor of 100 times smaller. This could imply that 'Oumuamua was heated to ∼100 K prior to our observations—either by our Sun, or before entering our solar system. 'Oumuamua, if propelled by water ice sublimation at 6 × 1025 molecules s−1 while producing only ≤9 × 1022 molecules s−1 of CO+CO2, must therefore have been previously devolatilized of these more volatile ices.
4.5. Possible Analogies
Unfortunately, we do not have pre-perihelion observations to compare to these post-perihelion observations to test the hypothesis that 'Oumuamua brightened during its perihelion passage. Further modeling of 'Oumuamua's outgassing—whether CO, CO2, or some other species—would be very beneficial.
A plausible analogy for such activity-produced resurfacing is the well-studied comet 67P, the target of the Rosetta mission. Keller et al. (2017) and Liao et al. (2018) showed that the nucleus of 67P was partially resurfaced through re-condensation of volatiles released from the nucleus; Liao et al. (2018) found that the deposition rate of water ice could be up to several microns in an hour near perihelion. While this does not correspond directly to low levels of activity and an albedo increase, as proposed here for 'Oumuamua, it is nevertheless evidence that activity can resurface small-body surfaces after perihelion passage, at the order of magnitude required for 'Oumuamua (1–10
Another possible analogy is the well-studied comet Shoemaker-Levy 9. Sekanina (1995) shows that after the breakup of this body from tidal forces exerted by Jupiter, many fragments appeared intrinsically brighter—as if fresh ice had just been revealed or deposited onto their surfaces. It is possible that the shape and/or rotation state of 'Oumuamua were affected by its passage near the Sun, and interior volatiles may also have been liberated onto the surface at the same time.
5. Conclusions
We observed interstellar body 'Oumuamua for 30 hr of integration time at 4.5
It is not clear what type of body in our solar system is the most similar to 'Oumuamua, as there are significant failures with both comets and primitive (D-type) asteroids as end-member analogs. One possible scenario that appears to explain many of the observed properties of 'Oumuamua, including our observations, is exposure or creation, from outgassing, of a fresh, icy, bright surface due to thermal reactivation during 'Oumuamua's close perihelion passage in 2017 September. However, due to the geometry of 'Oumuamua's passage through the solar system, there will be no more observations of this object, so it is likely that we will never know the true nature of this interstellar interloper.
This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA.
We thank the SSC Director for approving these DDT observations and the SSC staff for rapidly implementing these observations with their usual technical excellence. Part of this research was conducted at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. K.M. acknowledges support from NSF awards AST1413736 and AST1617015.
Facility: Spitzer(IRAC). -
Software: MOPEX (Makovoz et al. 2006), IRACproc (Schuster et al. 2006).
Footnotes
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To compute this average, we assume a uniform distribution of spin poles on the sphere of the body. This is obtained by uniform sampling in longitude and uniform sampling in the sine of the latitude. Now we compute the average of latitude knowing that sine of latitude is uniform. The average from −1 to 1 is zero (since this distribution is symmetric), but if we take only the northern hemisphere (for example) then we calculate , which is 327. We note that even under other assumptions of the average value, the deviation from our nominal solutions is insignificant in all cases.