Spectroscopy of Near-Earth Asteroids

Results of visible to near-infrared spectra and taxonomical classifications of the main belt asteroid 1866 Sisyphus and the Near-Earth asteroids 2008 HR3, 2008 JU2 and 1999 HF1 are presented. The data was obtained using the 2.4 meter Hiltner telescope at the MDM Observatory in Arizona. The result obtained for 1866 Sisyphus, of class S, agree with published data. Based on comparison with the spectra obtained for these asteroids and published spectra of other asteroids and on the SMASS taxonomical classification, the NEAs 1999 HF1, 2008 HR3 and 2008 JU2 belong to the Xe, Sr and V types respectively.

1. Introduction

NEAs are rather small objects, usually of the order of a few kilometers or less. It is therefore only when NEAs approach the Earth that they provide an oportunity for spectroscopic observations with the aim of taxonomical classification. The most widely used taxonomical classification was proposed by Tholen (1984). This classification was developed from broad band spectra, between 3100 and 10600 Å, combined with albedo measurements and based on the Eight-Color Asteroid Survey (ECAS) of 978 asteroids. There are 14 types in this classification with the three main categories being the C-group, the S-type and the X-group. The C-group are dark carbonaceous objects divided in B, F, G and C type. The S-type are silicaceous objects and the X-group are mostly metallic objects and is divided in three types: M, E and P. A more recent taxonomical classification was introduced by Bus & Binzel (2002). This classification was developed from broad band spectra between 4400 and 9200 Å without considering the albedos and based on the Small Main-Belt Asteroid Spectroscopic Survey (SMASS) of 1447 asteroids. This new classification attempts to preserve the Tholen taxonomy and resulted in 26 types with also three main categories: the C-group,the S-group and the X-group (see Appendix A).

NEAs are believed to be fragments of main-belt asteroids ejected on their current orbits by various mechanismsis but the relationships among these classes of bodies are not at present completely understood. The study of spectral reflectance of these objects can help us to impose additional constraints on the origin of these populations. Therefore two Near-Earth asteroids with unknow spectral type, 2008HR3 and 2008JU2, have been observed in order to increase our knowledge about the compositional distribution of these objects. 2008HR3 and 2008 JU2 have an absolute magnitude (H) of 24.821 and 22.789 which correspond to approximately 34 and 86 meters in diameter respectively. The NEA 2008 JU2 had an apparent magnitude (V) of 19.1 at the date of observation and the NEA 2008 HR3 was observed in two different nights with a V of 18.2 and 17.6 respectively. 2008 HR3 was specifically chosen because it has a high probability of being a fast rotator due to its small size. Photometry was obtained for this object on the same run but has not been reduced yet. Also two other asteroids with known spectral type were observed for comparison: 1866 Sisyphus and 199HF1. 1866 Sisyphus is a main belt asteroid approximately 11 Km in diameter that has been classified as an S-type and 1999HF1 is a NEA with H=14.413 that has been classified as a X:-type.

2. Observations

Table 1. Observation summary

Object	Date of Observation	Telescope / Instrument	V	H	# of Exp.	Exp. time	Solar analog
2008 HR3	May 9, 2008	2.4 meter Hiltner / CCDS	18.2	24.821	4	900 s	HD143436
2008 JU2	May 9, 2008	2.4 meter Hiltner / CCDS	19.1	22.789	4	900 s	HD143436
2008 HR3	May 10, 2008	2.4 meter Hiltner / CCDS	17.6	24.821	5	900 s	HD143436
1999 HF1	May 10, 2008	2.4 meter Hiltner / CCDS	15.4	14.413	5	900 s	HD143436
1866 Sisyphus	May 10, 2008	2.4 meter Hiltner / CCDS	16.9	12.343	6	900 s	HD143436

3. Reductions

A fringing pattern can be observed in the flat fields, this fringing of CCD occurs because of interference between the incident light and the light internally reflected at the interfaces between the layers of the CCD chip. An incandescent light bulb that produces a continuous spectrum is used to illuminate the slit in order to obtain the lamp flat fields. A continuum spectrum will be subjected to the same variations as the lamp flat field and therefore can be corrected with the lamp flat field image. In the other hand, the night sky is dominated by emission lines that create a fringe pattern caused by the combination of the interference for the different monochromatic lines. The observed fringe patterns therefore depend in the particular lines which are strong in the night sky and a lamp flat field obtained with a continuum source will not properly correct the sky regions as the fringing pattern wil not be the same.

When inspecting the images, the fringing pattern in the sky regions was neglegible, therefore the flat fields obtained with the continuum spectrum were just smoothed in order to have an appropiate flat field for the sky regions. Two different methods of flat-fielding were then done to the images in order to properly correct for the fringin pattern in the continuum spectrum and in the sky regions. For the first method, each lamp flat field was multiplied by the Sky to create a perfect flat to be applied to the images to correct the spectrum. In the second method, each lamp flat field was fit with a surface function using the pakage imsurfit and then multiplied by the Sky to create a perfect flat to be applied to copies of the images in order to have sky regions without any fringes. The images and their copies were then flat-fielded with their corrresponding perfect flats.

The packages identify, reidentify, fitcoords and transform were used to geometrically correct the data in order to have the spatial axis exactly along a column and the dispersion axis along a line. A wavelength calibration is also performed as part of this step in order to have a wavelength scale in the dispersion axis. This is done by identifying lines along the comparison spectrum and giving their corresponding wavelength. Then a fit of wavelength as a function of pixel number is obtained. The spectrum region from the images flat-fielded with the first method were then combined with the images flat-fielded with the second method into a single image. Then appal was used to extract a one-dimensional spectrum from the two-dimensional combined image. This is done by defining the extraction and the background windows, then the data within the extraction window is summed and the sky backround is substracted.

A spectrum with a wavelength scale instead of a pixel scale is obtained at this point since the wavelength calibration was also performed as part of the geometrical correction of the image. There is no need to trace or to dispersion correct the data at this point since a geometrically correction was already applied. Once obtained a 1-D spectrum the airmass was set to the middle value of the exposure and an extinction correction was applied. Then between four to six spectra (see Table 1) of each asteroid and three of the solar analog were co-added into a single spectrum. Finally, the spectrum of each asteroid is then divided by the spectrum of the solar analog.   

4. Results

Figure 1. Final spectrum from the data obtained for 2008 JU2 during the observing night of May 9. The x-axis is the wavelength in Å and the vertical scale is dimensionless and arbitrary.

Figure 2. Final spectrum from the data obtained for 2008 HR3 during the observing night of May 9. The x-axis is the wavelength in Å and the vertical scale is dimensionless and arbitrary.

Figure 3. Final spectrum from the data obtained for 2008 HR3 during the observing night of May 10. The x-axis is the wavelength in Å and the vertical scale is dimensionless and arbitrary.

Figure 4 Final spectrum from the data obtained for the main belt asteroid 1866 Sisyphus during the observing night of May 10. The x-axis is the wavelength in Å and the vertical scale is dimensionless and arbitrary.

Figure 5. Final spectrum from the data obtained for 1999 HF1 during the observing night of May 10. The x-axis is the wavelength in Å and the vertical scale is dimensionless and arbitrary.

All of the asteroids contained in the S-type have the common characteristic of a moderate red slope over the wavelength interval of 0.44 to 0.70 µm and a peak reflectance around 0.73 µm. Our spectrum for 1866 Sisiphus has a moderate red slope at 0.65 µm and a peak reflectance around 0.75 µm, which is consistent with an S-type. Another characteristic that distinguish the S-type asteroids but that is out of the our wavelength range is a 1 µm absorption feature. Based ont the above information and on the comparison with the spectrum from the data by Binzel et al.2004, we conclude that the spectrum obtained from our data matches the published data for this object and that 1866 Sisyphus is a S-type with a moderate steep slope at wavelengths shorter than 0.7 µm.

Figure 6. Reflectance spectra for the main belt asteroid 1866 Sisyphus. The x-axis is the wavelength in Å and the vertical scale is dimensionless and arbitrary. Our data points corresponding to the mean values for every 50 Å normalized to the unity at .701 µm are plotted in blue with their error-bars of standard deviation. Data points obtained by Binzel et al. 2004 normalized to the unity at .55 µm with their corresponding errors are shown in red.

Table 2. 1866 Sisyphus

Reference	Spectral Slope at 0.65 µm (% per.1 µm)	Spectral Slope at 0.80 µm (% per.1 µm)	Taxonomic Group
Binzel et al. 2004	12.4 ± 0.2	-9.3 ± 0.3	S-type
Data	13.5 ± 0.5	-6.4 ± 0.5	S-type

The NEA 1999 HF1 has been previously clasiffied as an X:-type by Binzel et al. 2004, where the colon denotes the uncertainty of the taxonomic assigment. Asteroids in the X-group have been described as generally featureless over the interval from 0.4 to 1. µm, with slopes that range from slightly to moderately red. They were divided in three types by Tholen (1984) based only on the albedo value, however Bus and Binzel (2002) found that the spectra of these asteroids were not uniformly featureless and the X-group was divided into four types: X, Xc, Xe, and Xk. The spectrum obtained for this NEA is shown in Figure 7a in the region of 0.6-0.925 µm with the spectra for the asteroids 5751 Zao, 3691 Beder, 3103 Eger and 2000 CK33 with spectral types X, Xc, Xe and Xk respectively for comparison. The spectrum obtained by Binzel et al. 2004 for 1999 HF1 is not included in Figure 9a since it was observed from 0.89 to 1.6 µm. The binned data points and one standard deviation error bars normalized to the unity at .701 µm are shown in blue. The spectral slopes at .65 µm and .8 µm obtained for the spectra are given in Table 3.

Figure 7a. Reflectance spectra for the NEA 1999 HF1 and the asteroids 5751 Zao, 3691 Beder, 3103 Eger and 2000 CK33 with spectral types X, Xc, Xe and Xk respectively. The x-axis is the wavelength in Å and the vertical scale is dimensionless and arbitrary. The spectrum obtained from our data normalized to the unity at .701 µm are shown in blue and the data points from Binzel et al. 2004 were normalized to the unity at .55 µm.

Table 3. X-group

Reference	Asteroid	Spectral Slope at 0.65 µm (% per.1 µm)	Spectral Slope at 0.80 µm (% per.1 µm)	Taxonomic Group
Binzel et al. 2004	2000 CK33	7.5 ± 0.3	6 ± 0.1	Xk-type
Binzel et al. 2004	5751 Zao	4.2 ± 0.1	-3.1 ± 0.2	X-type
Binzel et al. 2004	3691 Bede	3.0 ± 0.1	-2.1 ± 0.2	Xc-type
Data	1999 HF1	10.1 ± 0.1	-3.0 ± 0.4
Binzel et al. 2004	3103 Eger	9.7 ± 0.1	0.3 ± 0.1	Xe-type

By comparing the first half of the spectra we can see that the asteroids 5751 Zao and 3691 Bede have a nearly flat spectrum while the spectrum of 1999 HF1 has a moderate red slope similar to 3103 Eger and 2000 CK33. If we compare the second half, the spectrum for 1999 HF1 have a slightly blue slope while 2000 CK33 has a moderate red slope and 3103 Eger has a nearly flat spectrum. Based on the above comparison and on the descriptions for the X, Xc, Xe and Xk types given by the SMASS classification in Appendix A, we conclude that the spectrum for 3103 Eger is a better match to 1999 HF1 than the other spectra and therefore that 1999 HF1 is an Xe-Type. Figure 7b shows the spectrum obtained for 1999 HF1 and the near-infrared data obtained by Binzel et al. 2004 shifted in the vertical axis by -0.1 so that there is no overlap between the two sets of data points. In this figure can be seen how well the two data sets match each other with respect to the shape of the spectrum. With our data and that obtained by Binzel et al. 2004 now there is a spectrum for 1999 HF1 in the wavelength range of 0.6 to 1.6 µm.

Figure 7b Spectra for the NEA 199 HF1. The x-axis is the wavelength in Å and the vertical scale is dimensionless and arbitrary. The spectrum obtained from our data normalized to the unity at .701 µm are shown in blue and data points from Binzel et al. 2004 shifted in the vertical axis by -0.1 so that there is no overlap between the two sets of data points are shown in red.

The NEA 2008 HR3 is shown in Figures 8 and 9 for two different observing nights. The spectra obtained resembles the same shape in both observing nights and both of them have peaks around .73 µm. The spectrum of the NEA 2008 HR3 has a similar shape to the spectra of asteroids in the S-group. Therefore the spectrum obtained for this NEA is shown in Figure 10 in the region of 0.6-0.925 µm with the spectra for the asteroids 1866 Sisyphus, 2062 Aten,2201 Oljato, 2335 James and 3674 Erbisbuhl with spectral types S, Sr, Sq Sa and Sk respectively for comparison. The spectral slopes at .65 µm and .8 µm obtained for the spectra are given in Table 4.

Figure 8. Binned spectrum for the NEA 2008 HR3 from the data obtained in May 10. The x-axis is the wavelength in Å and the vertical scale is dimensionless and arbitrary. Data points corresponding to the mean values for every 50 Å normalized to the unity at .701 µm are plotted in blue with their error-bars of standard deviation.

Figure 9. Binned Spectrum for the NEA 2008 HR3 from the data obtained in May 9. The x-axis is the wavelength in Å and the vertical scale is dimensionless and arbitrary. Data points corresponding to the mean values for every 50 Å normalized to the unity at .701 µm are plotted in blue with their error-bars of standard deviation.

Figure 10. Spectra for the NEA 2008 HR3 from the data obtained in May 9 and for the asteroids 1866 Sisyphus, 2062 Aten,2201 Oljato, 2335 James and 3674 Erbisbuhl with spectral types S, Sr, Sq Sa and Sk respectively. The x-axis is the wavelength in Å and the vertical scale is dimensionless and arbitrary. The spectrum obtained from our data normalized to the unity at .701 µm are shown in blue and data from Binzel et al. 2004 were normalized to the unity at .55 µm.

Table 4. S-group

Reference	Asteroid	Spectral Slope at 0.65 µm (% per.1 µm)	Spectral Slope at 0.80 µm (% per.1 µm)	Taxonomic Group
Binzel et al. 2004	3674 Erbisbuhl	7.5 ± 0.1	-13.5 ± 0.2	Sk-type
Binzel et al. 2004	1866 Sisyphus	12.4 ± 0.2	-9.3 ± 0.3	S-type
Binzel et al. 2004	2201 Oljato	6.1 ± 0.2	-15.1 ± 0.2	Sq-type
Data	2008 HR3	14.7 ± 1.6	-20.2 ± 0.6
Binzel et al. 2004	2062 Aten	11.9 ± 0.1	-16.8 ± 0.1	Sr-type
Binzel et al. 2004	2335 James	15.7 ± 0.2	-14.9 ± 0.6	Sa-type

By comparing the first half of the spectra we can see that the asteroids 2201 Oljato and 3674 Erbisbuhl have a slight reddish slope while the spectrum of 2008 HR3 has a moderate red slope similar to 1866 Sisyphus, 2062 Aten and 2335 James. If we compare the second half, the spectrum for 1866 has a moderate blue slope in comparison with the deep absorption feature longward of 0.75 µm of 2008 HR3, 2062 Aten and 2335 James. If we look carefully, although the spectrum of 2335 James looks very similar to tha of 2008 HR3, the apsorption feature is not as deep as those of 2008 HR3 and 2062 Aten. Based on the above comparison and on the descriptions for the S, Sr, Sq, Sa, and Sk types given by the SMASS classification in Appendix A, we conclude that the spectrum for 2008 HR3 is an Sr-Type.

The NEA 2008 JU2 is shown in Figure 11. The spectrum of the NEA 2008 JU2 also resembles the shape of the spectra ofasteroids in the S-group but it is sharply peaked as a V-type. The spectrum obtained for this NEA and a fit to the spectrum are shown in Figure 12 in the region of 0.6-0.85 µm with the spectra for the asteroids 2000 JQ6, 4066 Magellan, 3753 Cruithne, 2062 Aten and 2335 James with spectral types R, V, Q, Sr and Sa respectively for comparison. The spectral slopes at .65 µm and .8 µm obtained for the spectra are given in Table 5.

Figure 11. Binned Spectrum for the NEA 2008 JU2. The x-axis is the wavelength in Å and the vertical scale is dimensionless and arbitrary. Data points corresponding to the mean values for every 50 Å normalized to the unity at .701 µm are plotted in blue with their error-bars of standard deviation.

Figure 12. Spectra for the NEA 2008 JU2 and for the asteroids 2000 JQ6, 4066 Magellan, 3753 Cruithne, 2062 Aten and 2335 James with spectral types R, V, Q, Sr and Sa respectively. The x-axis is the wavelength in Å and the vertical scale is dimensionless and arbitrary. The spectrum obtained from our data normalized to the unity at .701 µm are shown in blue and data from Binzel et al. 2004 were normalized to the unity at .55 µm.

Table 5. S-group and V-type

Reference	Asteroid	Spectral Slope at 0.65 µm (% per.1 µm)	Spectral Slope at 0.80 µm (% per.1 µm)	Taxonomic Group
Binzel et al. 2004	2335 James	15.7 ± 0.2	-14.9 ± 0.6	Sa-type
Binzel et al. 2004	2062 Aten	11.9 ± 0.1	-16.8 ± 0.1	Sr-type
Binzel et al. 2004	2000 JQ6	10.7 ± 0.1	-28.0 ± 0.2	R-type
Binzel et al. 2004	3753 Cruithne	7.2 ± 0.1	-17.2 ± 0.4	Q-type
Data	2008 JU2	14.6 ± 1.6	-32.3 ± 2.4
Binzel et al. 2004	4055 Magellan	13.7 ± 0.2	-43.5 ± 0.02	V-type

   By comparing the first half of the spectra we can see that the spectral slope of the asteroids is steep in comparison with the asteroid 3753 Cruithne. If we compare the second half, the spectrum for 2062 Aten and 2335 James do not have an absorption feature as deep as 2008 JU2, 200 JQ6 and 4055 Magellan. Between 4055 Magellan and 200 JQ6, 4055 Magellan is more similar to the 2008 JU2 spectrum since it is sharply peaked whether 2000 JQ6 is quite broad at the maximum. Based on the above comparison and on the descriptions for the Sr, Sa, R, V and Q types given by the SMASS classification in Appendix A, we conclude that the spectrum for 2008 JU2 is a V-Type.   

5. Discussion

Although there are some discrepancies in the quality of the data for the NEA 2008 HR3 between the two observing nights, the spectrum resembles the same shape in both nights. Due to the similarity of the shape of the 2008 HR3 spectrum with the shape of the spectra in the S-group it was compared to the spectrum of 1866 Sisyphus and the spectra of other asteroids in the S-group. After comparing them and once again based on the SMASS classification, it was determined that 2008 HR3 is an Sr-type. Similarly the spectrum of the NEA 2008 JU2 was compared to others asteroids in the S-group that have a similar shape in the spectrum. In this case it was also compared to a V-type which is not part of the S-group due the characteristic of a sharply peaked maximum similar to that observed for 2008 JU2. Based on the SMASS classification and the different characteristics of the spectra it was determined that the NEA 2008 JU2 is a V-type.

Based on our results a spectrum in the wavelength range of 0.6 to .97 µm can be properly classified into a corresponding taxonomical type although some characteristic features that distinguish the different classes are outside of that range. Many features in the different types of the taxonomical classification are found near 0.55 µm and 1 µm. Although from our data we were able to identified the different taxonomical classes, a better reduction should be done again in order to make sure that sky subtraction from the summed spectrum has been properly achieved.   

Appendix A

Figure A1. Key showing all 26 SMASS taxonomic classes. Spectral slope increases from left to right. Reproduced from Bus (1999).

Table A1. Description of the classes A to Ld in the SMASS classification. Reproduced from Bus (1999).

Table A2. Description of the classes O to Sr in the SMASS classification. Reproduced from Bus (1999).

Table A3. Description of the classes T to Xk in the SMASS classification. Reproduced from Bus (1999).

Acknowledgements

References