Density plots in R: differences with and without $

Question

I have a dataframe d like this

head(d) 
       Id Group    Index
1 ID_key1     F 38.69361
2 ID_key2     A 34.01609
3 ID_key3     A 31.21704
4 ID_key4     A 30.72222
5 ID_key5     F 39.52463
6 ID_key6     A 28.98853

and I want to plot a density plot of the variable Index grouping observations by the variable Group

I did it using the following code

ggplot(d, aes(x= Index)) + geom_density() + facet_grid(Group~ .)

and I obtained this density plot

However, I noticed that if I use the dollar $ to select the data for the plot within the previous line of code, like this

ggplot(d, aes(x = d$Index)) + geom_density() + facet_grid(d$Group~ .)

the resulting graph is totally different:

I always thought that the two forms were equivalent. Why there is this difference between the two plots?