A New Photometric Study of Ap and Am Stars in the Infrared

In this paper, 426 well known confirmed Ap and Am stars are photometrically studied in the infrared. The 2MASS, Wide-field Infrared Survey Explorer (WISE), and IRAS data are employed to make analyses. The results in this paper have shown that in the 1–3 μm region over 90% Ap and Am stars have no or little infrared excesses, and infrared radiations in the near-infrared from these stars are probably dominated by the free–free emissions. It is also shown that in the 3–12 μm region, the majority of Ap stars and Am stars have very similar behavior, i.e., in the W1–W2 (3.4–4.6 μm) region, over half of Ap and Am stars have clear infrared excesses, which are possibly due to the binarity, the multiplicity, and/or the debris disk, but in the W2–W3 (4.6–12 μm) region they have no or little infrared excess. In addition, in the 12–22 μm region, some of Ap stars and Am stars show the infrared excesses and infrared radiations for these Ap and Am stars are probably due to the free–free emissions. In addition, it is seen that the probability of being the binarity, the multiplicity and/or the debris disk for Am stars is much higher than that for Ap stars. Furthermore, it can be seen that, in general, no relations can be found between infrared colors and spectral types either for Ap stars or for Am stars.


Introduction
Chemical peculiar (CP) stars are mainly found among B and A stars on the upper main sequence. Preston (1974) first used the term "Chemical Peculiar Star" (CP Star) for thesekinds of stars and gave the definition of CP stars as follows."The CP stars are identified by the presence of anomalously absorption lines of certain elements in their spectra." Hack (1981) further pointed out that the spectra of CP stars present an anomalous intensity of the lines of several elements, which can be interpreted as a surface phenomenon, an effect of an anonymous chemical composition of their atmospheres and not a consequence of the different evolutionary stage or membership in different populations. The important differences of CP stars defined by Preston (1974) with other "chemically peculiar stars," such as Ba II stars, R stars, S stars, and chemically peculiar late-type dwarfs is that the CP stars are on the upper main sequence, but other "chemically peculiar stars" are on the horizontal branch, the red giant branch or the asymptotic giant branch in the H-R diagram, and in consequence, the spectral types of CP stars are not later than F, but other "chemically peculiar stars" have more later spectral types.
CP stars can be classified into several subclasses according to Preston (1974) and Schnell (2008): 1. CP1 = Am stars (the metallic line star) with the high binary frequency and without strong magnetic field; 2. CP2 = Ap stars with strong magnetic field and the low binary frequency; 3. CP3 = HgMn stars, most of them have no magnetic field; 4. CP4 = He weak stars, some of them have detectable magnetic field.
Although many kinds of CP stars are discovered up to date (besides the stars above, λ Boo stars, etc., are also included), the number of Am and Ap stars is over 97% among all CP stars according to Renson & Manfroid (2009). Therefore, the study of Am and Ap stars are very important to understand the nature of CP stars.
At different times, Preston (1974), Hack (1981), Faraggiana (1987), Cowley & Bord (2004), and Schnell (2008) gave good reviews for the study of CP stars. In addition, Michaud et al. (2015) recently presented amore advanced review for CP stars in their book "Atomic Diffusion in Stars." Here we do not want to give the unnecessary descriptions and one can refer tothese references for more detail.
The earlier infrared study of CP stars can be dated back to 1978. The workshop named"Ap-stars in the infrared" held in 1978 summarized the early infrared study of Ap stars. In this workshop, some infrared observations of Ap stars, photometric properties of Am stars in the infrared, and angular diameters and effective temperatures of Ap stars were presented and discussed (Weiss & Kreidl 1979). Later,Groote & Kaufmann (1981) observed 82 Ap/Bp stars in JHKLM bands. They found that no infrared excesses can be found in the JHK bands. They also found, however, that60% of samples have the infrared excess in the M band (4.8 μm). They also declared that the infrared excess in the M band appears more frequently for earlier type stars and thatthere is a decrease of the maximum excess with the spectral type from B0 to F0. Groote & Kaufmann (1983) further observed 105 CP stars in JHKLM bands and confirmed infrared excess in the M band for many stars. Groote & Kaufmann (1984) also claimed that some CP stars even show slightly infrared excess in theL band (3.6 μm), but the reason of the infrared excess is still unknown. However, Bonsack & Dyck (1983) observed 22 CP stars and 18 normal (non-peculiar) comparison stars in JHKL′M bands. They found that for these two groups of stars infrared colors are similar. Kroll (1987) supported the conclusion from Bonsack & Dyck (1983) and agaist the conclusions from Groote & Kaufmann (1983, 1984, i.e., no infrared excess can be found for CP stars. Therefore, infrared observation is necessary for CP stars beyond 5 μm. Then, Kroll (1987) analyzed IRAS data for 40 CP stars and found that CP2 and CP3 stars have no infrared excess, but two CP4 stars show the circumstellar dust like Be stars. After the1990s,the infrared study of CP stars becamerare, only Shylaja & Ashok (2002) reported the observations of 22 Am stars in JHKLM and IRAS bands. They found that, in general, all Am stars have infrared excesses, and the 12 μm fluxes tend to have infrared excesses. They further pointed out that infrared excess for Am stars is possibly due to the circumstellar dust, and in 25/60μm may also be due to the free-free emission from the stellar wind. Recently Herdin et al. (2016) reported the 2MASS observation for rather large number of CP stars. They found that no differences in the astrophysical parameters derived from 2MASS are found between CP stars and normal (non-peculiar) stars.
Some information about infrared studies of CP stars since 1981has been summarized in Table 1. Renson et al. (1991) published the first catalog of Ap and Am stars containing 6684 objects including probable and suspected ones. In 2009, Renson & Manfroid (2009) modified this catalog and published the General Catalog of Ap and Am stars in which 8205 known, probable and suspected Ap (3652), Am (4299), and HgMn (162) stars are included. As Renson & Manfroid (2009) described, in this catalog, the category "Ap star" is taken in the broadest sense. It also includes early Bp stars and late Bp stars. Similarly, it includes the Fm stars under the Am banner. However, as they claimed, only 426 samples are "well known confirmed Ap, Am,and HgMn stars," but they did not give any criteria for these "well known confirmed Ap, Am, and HgMn stars." These 426 samples include 180 Ap stars, 69 late Bp stars, 19 early Bp stars, 116 Am/Fm stars, and 42 HgMn stars.
It is noted that the infrared photometric study of Ap and Am stars should be reconsidered nowfor several reasons.(1) The number of stars studied in most previous works is quite small. Therefore, in order to obtain the reliably statistical result, the number of Ap and Am stars studied should be increased.(2) Recently, the new space mission, the Wide-field Infrared Survey Explorer (WISE) mission, has included the observational range of 3-22 μm,which covers the observational wavelength range just between the 2MASS and IRAS and includes the observation around 5 μm so that the photometric study of Ap and Am stars in the full infrared wavelengths from 1-60 μm can be made to further outline infrared properties for Ap and Am stars. It is also possible to clearly reveal whetherinfrared excesses around 5 μm for Ap and Am stars are real or not.
The WISE mission has completed theall sky survey in the mid-infrared (Wright et al. 2010). WISE performed observations in four bands: W1 (3.4 μm), W2 (4.6 μm), W3 (12 μm),and W4 (22 μm), and WISE all-sky data released in 2012 March 14. This survey extended the 2MASS All Sky Survey into the mid-infrared and connected with the far-infrared observation by IRAS.
In this paper, we use the General Catalog of Ap and Am and stars by Renson & Manfroid (2009) as our basic working sample to systematically study Ap and Am stars in the infrared based on 2MASS, WISE, and IRAS data. Note that, to avoid the possible wrong classification, the suspected and probable stars are not included, and only "well known confirmed stars" are studied in this paper.
In addition, some information about the wavelengths and widths in various infrared bands referred to in this paper are collected in Table 2 as the references.

Data Processing
In the General Catalog of Ap and Am stars (Renson & Manfroid 2009) 8265 stars are included in which, as Renson & Manfroid (2009) described, only 426 stars are of the"well known confirmed sample." We take these 426 stars as our working sample. 2MASS JHK No differences in the astrophysical parameters can be found for both groups of stars          The cross-identifications of 2MASS/WISE counterparts for all Ap, Am, and HgMn stars listed in this paper are made from Cutri et al. (2012) by using the radius of 2 arcsec. All 426 Ap, Am, and HgMn stars have 2MASS and/or WISE counterparts,which are listed in Table 3. The contents in Table 3 are(1) the star number in this paper,(2) the star number from the General Catalog of Ap and Am stars (Renson & Manfroid 2009),(3) the star name (HD or others),(4) the star position in the epoch of 2000 from the General Catalog of Ap and Am stars,(5) the spectral types and CP star types of Am, Ap, and HgMn from Skiff (2009-), All-Sky Compiled Catalog of 2.5 million stars (ASCC) Kharchenko & Roeser (2009) or Renson & Manfroid (2009),(6) the Galactic extinction coefficient Av,(7) magnitudes with measurement uncertainties in 2MASS JHK bands, and if the empty is in the uncertainty, it means the upper limit value, and(8) magnitudes with measurement uncertainties in four WISE bands, and if the empty is in the uncertainty, it means the upper limit value.
The Galactic extinction corrections for 2MASS/WISE data should be made before discussions below. The Galactic extinction laws used are from Schlegel et al. (1998) for 2MASS data and from Yuan et al. (2013) for WISE data respectively. The Galactic extinction coefficient Av listed in Table 3 Kharchenko & Roeser (2009),while Bo-Vo is from Cramer (1984) and/or Bessell (1990).
The cross-identifications of IRAS counterparts are made according to the positional error ellipse of the source, because it has a 95% confidence level (IRAS Explanatory Supplement 1988). Finally, 202 stars are found to have the IRAS counterparts from IRAS PSC/FSC, which is listed in Table 4. The contents in Table 4 are(1) the star number in this paper,(2) the IRAS PSC/FSC name,(3) and the fluxes in Jy for 12, 25, and 60 μm, and "q" shows the data quality, if q=1 means the upper limit value. From Table 4, it can be found that only six stars have good quality data in all 12, 25, and 60 μm, and 46 stars have good quality data in 12 and 25 μm.
We also want to search the AKARI FIS (Kawada et al. 2007) counterparts of stars listed in Table 3. However, only four stars have the AKARI FIS counterparts, but no one has good data in more than twobands. Therefore, in the sections below, the AKARI FIS data are not discussed.
It is noted from Table 3 that 42 HgMn stars are included. To concentrate on Ap and Am stars, these HgMn stars are not included in the following discussion. In addition, from Table 3, it can be seen that some late Bp and early Bp stars are also included for the following discussion.

Two-color Diagram From 2MASS, WISE,and IRAS
By using 2MASS, WISE,and IRAS data from Tables 3 and 4, several two-color diagrams can be presented to show infrared properties for Ap and Am stars. In these two-color diagrams,if stars are located in the lower-left region, those stars have small or no infrared excesses and high color temperatures, while if stars are in the upper-right region, those stars have large infrared excesses and low color temperatures.
In order to show infrared properties clearly, the blackbody distributions and the power-law distributions are also presented in these two-color diagrams.
The power-law distribution is derived in such a way that in a certain wavelength range Δλ=λ 2 -λ 1 (λ 1 <λ 2 ), α is the where m(λ 1 ) and m(λ 2 ) are magnitudes at wavelengths λ 1 and λ 2 respectively. S λ1 and S λ2 are absolute flux calibrations at wavelengths λ 1 and λ 2 respectively. For example, taking m(λ 1 ) -m(λ 2 ) as H-K and J-H,respectively, the power-law distribution can be determined in the 2MASS two-color diagram. The blackbody distribution is derived according to the calculation of the Plank function in two wavelengths and a certain temperature around this wavelength interval, i.e., where B λ1 and B λ2 are the Plank functions with same temperature T at λ 1 and λ 2 ,respectively, and S λ1 and S λ2 are absolute flux calibrations at wavelengths λ 1 and λ 2 respectively. For example, taking m(λ 1 )-m(λ 2 ) as H-K and J-H,respectively, the blackbody distribution can be determined in the2MASS two-color diagram. In other two-color diagrams, similar procedures can be made to present the blackbody distribution and the power-law distribution.
Note that if stars are near the blackbody distribution in these two-color diagrams, the infrared radiations of those stars are mainly due to thermal emissions, while if stars are near the power-law distribution, the infrared radiations of those stars are mainly due to free-free emissions.
Note that in the following parts of this paper the Galactic extinction corrections are made for all 2MASS and WISE data according to the method mentioned in Section 2.
In addition, in these two-color diagrams, the distributions of the Ap star, the Am star, the late Bp star, and early Bp star are separately presented in order to clearly show their properties. Furthermore, all data points are shown with the error bars in the X axis and Y axis.

2MASS Two-color Diagram
The (J-H) versus (H-K ) diagram is presented in Figure 1. It is seen from Figure 1 that (1) the majority of Am stars are located in the range of −0.1< H-K<0.1 and −0.1< J-H<0.2, while the majority of Ap stars are located in the range of −0.1<H-K<0.1 and −0.2<J-H<0.2. In addition, late Bp stars and early Bp stars are located in the range of −0.1<H-K<0.1 and −0.2<J-H<0.1. Thisimplies that no or little infrared excess can be found for these four groups of stars. Because Bessell & Brett (1988) pointed out that the average intrinsic colors are −0.04<H-K<0.03 and −0.05<J-H<0.09 for B and A stars, The result here is coincident with that from all previous works mentioned in Section 1.(2) Several Am stars are located in the region of H-K>0.2 and J-H>0.2,indicative of some infrared excesses, while almost no Ap stars and Bp stars are in this region. It implies that, statistically, infrared excesses for Am stars are larger than thosefor Ap and Bp stars. Nevertheless, over 90% ofAm, Ap, and Bp stars have no infrared excesses in the JHK bands.(3) Most Am stars and Ap stars, and almost all Bp stars, are around the power-law lines or located in the right-down region of the power-law lines. This implies that infrared radiations in the near-infrared from these stars are mainly dominated by free-free emissions.

WISE Two-color Diagrams
The (W2-W3) versus (W1-W2) diagram is presented in Figure 2. It can be seen from Figure 2 that (1) in the 3-12 μm region the majority of Am stars and Ap stars have very similar behavior, i.e., they are distributed in the region of −0.1<W1-W2<0.8 and −0.8<W2-W3<0.1, and the majority of late Bp stars are located in the region of −0.1<W1-W2<0.5 and −0.5<W2-W3<0.0,while the majority of early Bp stars are located in the region of −0.1<W1-W2<0.3 and −0.5<W2-W3<0.1. Thismeans that, in the W2-W3 color, they have no infrared excess while in the W1-W2 color more than half of theAp and Am stars, and several Bp stars have clear infrared excesses.(2) Over 40 Am stars, 20 Ap stars, and several Bp stars even have W1-W2>0.3 indicative of strong infrared excesses, because Bessell & Brett (1988) indicated that for A and B stars the average intrinsic color L (3.45 μm)-M (4.75 μm) is around 0.00-0.02. This result supports the previous suggestions by Groote & Kaufmann (1981, 1984, i.e., 60% of samples have the infrared excess in the M band (4.8 μm).(3) Most Am stars, Ap stars, and late Bp stars are located in the right-down region from the blackbody line indicative of their infrared radiations mainly due to thermal emissions, while most early Bp stars are in the left-upper region from the power-law distribution, indicative of their infrared radiations mainly due to free-free emissions.(4) The two Ap stars and three early Bp stars have strong infrared excess in the W2-W3 color with W2-W3>0.15 and located in the leftupper region from the power-law distribution indicative of strong free-free emissions.
The (W3-W4) versus (W2-W3) diagram is plotted in Figure 3. It is clearly shows that (1) for the majority of Am stars, Ap stars, and Bp stars,infrared excesses are not found in the W2-W3 color as already shown in Figure 2. However, several Am stars and about 40 Ap stars, 15 late Bp stars, and 3 early Bp stars show the infrared excesses with W3-W4>0.2, which is coincident with the result from Shylaja & Ashok (2002) in the similar wavelength region from IRAS. (2) Most Am stars, Ap stars, and Bp stars are located in the left-upper region from the power-law line indicative of the infrared radiation probably due to free-free emissions, while still some Am, Ap stars, and Bp stars are in the right-down region from the blackbody line indicative of their infrared radiations mainly from thermal emissions.(4) The distributions of these four groups of stars, in particular, the distributions of Ap stars and late Bp stars, in the (W3-W4) versus (W2-W3) diagram look rather strange. It can be seen that the majority of the sources have no infrared excess with the color dispersed in the region of −0.8 < W2-W3<−0.2 in the W2-W3 color and the W3-W4 colors are around 0.0, while for some sources with W2-W3>−0.2 their W3-W4 colors are spread in the broad range and some of them do have the infrared excess. These distributions are seenmore clearlyfor Ap and Bp stars.
Thisimplies that most sources with the infrared excess in the 12-22 μm (the W3-W4 color) can be found in sources with redder W2-W3 color (though they have no the infrared excess in the W2-W3 color).

IRAS Two-color Diagram
From Table 4, it can be seen that only six stars have all good data in 12, 25, and 60 μm;therefore, 46 stars with good data in 12 and 25 μm and upper limit data in 60 μm are also employed to make analyses.
The ( Figure 4. In Figure 4, the bold circle indicates the star with good quality data in all 12, 25, and 60 μm, while the thin circle without the error bar in the Y axis indicates the star with good quality data in 12 and 25 μm but upper limit data in 60 μm. It is

The Stars with the Infrared Excess in the W1-W2 Color
As shown in Section 3.2 and Figure 2, we found that many Ap and Am stars indeed have the infrared excesses in the W1-W2 color. This result supports the conclusion by Groote & Figure 2. WISE two-color diagram, (W2-W3) vs. (W1-W2), for Am, Ap, late Bp, and early Bp stars in this paper. The blackbody (BB) and the power-law (PL) distributions are also shown. Kaufmann (1981Kaufmann ( , 1983Kaufmann ( , 1984: many Ap and Am stars have infrared excess in the M band (4.8 μm). However, Groote & Kaufmann (1984) also pointed out that the reason for sources with the infrared excess in the M band is still unknown. Even up to now, no one has discussed the reason forthe infrared excess in the M band for these Ap and Am stars.
In order to reveal the reason of the infrared excess in W1-W2 color for some Ap and Am stars, we have taken the stars with W1-W2>0.1 (in fact, W1o-W2o, i.e., after the Galactic extinction correction) and listed them in Table 5 in which 75 Ap stars (including Bp stars) and 69 Am stars (including Fm stars) are presented. The columns in Table 5 are(1) the star number, the HD number, and the spectral classification,(2) the magnitudes in W1, W2,and W3 bands, and the Galactic extinction coefficient Av from Table 3,(3) the derived (W1-W2)o,and(4) the wavelength with the peak infrared excess, λpeak, and the average infrared excess within 2-60 μm from the Infrared excesses in Hipparcos stars by McDonald et al. (2012). In addition, we collected some information from the references listed in this table to check the status of the stars listed.
The important result we found is that, except for one star (No.066=HD 24712), all stars with W1-W2>0.2 in Table 5 are listed in theA catalog of multiplicity among bright stellar systems by Eggleton & Tokovinin (2008) (2014). This result indicates that, except for the source, No. 066, all stars with W1-W2>0.2 (including 42 Ap stars and 51 Am stars) are in the binary system, the multiplicity system or the debris disk system. The status of the sources with 0.1<W1-W2<0.2 is that 26 Ap stars, out of 30 and 13 Am stars, out of 18, are listed in the reference above. This result also indicates that those 26 Ap stars and 13 Am stars are also in the binary system, the multiplicity system or the debris disk system. Therefore, we founda total of68 Ap stars and 64 Am stars to be in the binary system, the multiplicity system or the debris disk system from the W1-W2 color.
On the other hand, we also checked Ap and Am stars with W1-W20.0, i.e., these stars have noinfrared excess. We found that none of thesestars arelisted in theA catalog of multiplicity among bright stellar systems and other references mentioned above.
Therefore, we can conclude that the reason of the infrared excess in W1-W2 color for some Ap and Am stars is mainly due to the influence from the binary,the multiplicity, or the debris disk. From Table 5, it is seen that the majority of Ap and Am stars have the wavelength with the peak infrared excess at 4.6 μm (the WISE W2 band), which again confirmed the distributions are also shown. Note that the bold circle indicates the star with good quality data in all 12, 25, and 60 μm, and the thin circle indicates the star with good quality data in 12 and 25 μm, but the upper limit data in 60 μm.     (2008)  (1) The source with " * " is listed in Table 5.
(2) The reference notes are the same as those of Table 5. Note.
(2) The source with "#" is listed in Table 6. (3) The reference notes are the same as for Table 5. conclusion of the infrared excess in M band by Groote & Kaufmann (1981), Groote & Kaufmann (1983), andGroote & Kaufmann (1984.

The Stars with the Infrared Excess in the J-H Color
We also checked the status for 15 Ap stars and 21 Am stars with J-H>0.1 (in fact, Jo-Ho, i.e., after the Galactic extinction correction). The result is listed in Table 6. The structure of Table 6 is the same as that of Table 5. It is seen that only one Ap star, No.407=HD 21363 is not listed in the references above. It means that their infrared excess in the nearinfrared is also mainly attributed to the multiplicity/binary/ debris disk. In addition, it can be seen from Table 6 that the majority of Ap/Am stars with the infrared excess in the nearinfrared also have the infrared excess in the W1-W2 color.

The Stars with the Infrared Excess in the W3-W4 Color
It can be seen from Figure 3 that many Ap and late Bp stars and some Am and early Bp stars have the infrared excess in the W3-W4. In order to check the status for these stars, we have taken the samples with W3-W4>0.3 in which 33 Ap stars and three Am stars are included. All stars with W3-W4>0.3 are listed in Table 7. The structure of Table 7 is the same as that ofTable 5. It is found that 17 Ap starsout of 33 and two Am starsout of threeare listed in the references above also indicating the infrared excess due to the multiplicity or the binary. In addition, 16 Ap stars and one Am starare not listed in the references above. The results may imply that their infrared excess beyond 12 μm is not mainly due to the influence from the binary/multiplicity. As Shylaja & Ashok (2002) pointed out, their infrared excesses in these wavelengths are possibly due to the circumstellar dust.

Discussion on the Probability of the Binary/Multiplicity
From Section 4,we can seethat due to the multiplicity/binary/ debris disk, 99 Ap stars and 87 Am stars have the infrared excesses in the 2MASS and/or WISE bands. Thus it can clearly be seen that the probability of being the binary/multiplicity system for Am stars is much higher than that for Ap stars. Because 268 Ap stars (including Bp stars) and 116 Am stars (including Fm stars) are listed in Table 3 in this paper, the probability of being the binary/multiplicity for Ap stars and Am stars are 37% and 75% respectively. It can be seen that the probability of being the binary/multiplicity for Am stars is double, such as for Ap stars. In comparison, some previous estimation of being the binary/ multiplicity for Ap stars and Am stars is shown in Table 8. It is seen that our results are higher than previous ones.

Relations between Infrared Colors and Spectral Types
Groote & Kaufmann (1981) observed 82 Ap/Bp stars in JHKLM bands and declared that the infrared excess in the M band appears more frequently for earlier type stars and there is a decrease of the maximum excess with the spectral type from B0 to F0.  To check this result, we attempt to find possible relations between infrared colors and spectral types for Ap/Am stars, not only in theM band, but also in all bands we studied in this paper.
The diagrams of the spectral type versus the J-K color are presented in Figure 5 for Am stars in the left panel and for Ap stars in the right panel respectively. It can be seen from Figure 5 that the J-K color appears to increase for the late Ap stars, but not for Am stars. As Bessell & Brett (1988) show, this is a normal result for the intrinsic colors of stars. In addition, no relation between the maximum excess and the spectral type can be found for Ap stars and Am stars.
The diagrams of the spectral type versus the W1-W2 color are presented in Figure 6 for Am stars in the left panel and for Ap stars in the right panel respectively. It can alsobe seen from Figure 6 thatno relation between the W1 (3.4 μm)-W2 (4.6 μm) color and thespectral type can be found foreitherAp stars or Am stars. This result is contrary to the result from Groote & Kaufmann (1981).
The diagrams of the spectral type versus the W2-W3 color are presented in Figure 7 for Am stars in the left panel and for Ap stars in the right panel respectively. It can be seen from Figure 7 that, statistically, no relation between the W2-W3 color and the spectral type can be found either for Ap stars or Am stars. However, the maximum infrared color appearsin the earlier type stars for Ap stars,whilethe minimum infrared color appearsin later type stars for Am stars.  The diagrams of the spectral type versus the W3-W4 color are presented in Figure 8 for Am stars in the left panel and for Ap stars in the right panel respectively. It can be seen from Figure 8 that, statistically, no relations between the W3-W4 color and the spectral type can be found either for Ap stars or Am stars. It is only worth to note that for Ap stars the earlier type ones have larger infrared colors.
On the whole, no relations can be found between the infrared colors and the spectral types either for Am stars or for Ap stars in the 2MASS and WISE region.

Summary
In this paper, 426 well known confirmed Ap and Am stars are photometrically studied in the infrared. The 2MASS, WISE,and IRAS data are employed to make analyses. Our result in this paper shows that in the W1 (3.4 μm)-W2 (4.6 μm) color, over half of Ap and Am stars indeed have infrared excesses. The conclusion of the infrared excess in the M band (4.8 μm) for many CP stars by Groote & Kaufmann (1981, 1984 is confirmed by this result. The important result we found is that infrared excesses in the W1-W2 color for these stars are mainly due to the influences of the binarity, the multiplicity, and/or the debris disk. However, statistically, either majority of Ap stars or majority of Am stars have no infrared excesses in the remaining infrared colors including the J-K, W2-W3, W3-W4 colors, and IRAS bands. In addition, it is seen that the probability of being the binarity, the multiplicity and/or the debris disk for Am stars is much higher than that for Ap stars. It is also seen that generally, no relations can be found between infrared colors and spectral types either for Ap stars or for Am stars in the 1-60 μm region.